Some speedlight reviews are linked on the sites main page.
A large chart of specs of a couple dozen speedlight models are on the next page.
Comparing flash power levels with Guide Number Rating.
This article is about choosing a speedlight for the hot shoe. Not specifically which, but rather about the big questions, to help think about what you need.
The speedlight flash can be used on the camera hot shoe, either as direct flash or as bounce flash, either as the single main light, or as fill flash, possibly to lighten harsh dark shadows in bright sun. The flash can be used off-camera, maybe in umbrellas, maybe multiple flash units, maybe for studio portraits or for tabletop or macro work. Or the speedlight can be used to stop fast motion of water drop splashes or hummingbird wings.
The first big concern is "How will you use the flash?". What do you need it to do? For any time the light is dim or harsh of course, but there are several options, and so much that can be done. The question is: Will you? What features do you need?
This was in normal daylight patio shade ambient (not bright, but far from dark). The big point here is that ISO 400, f/16, 0.8 second was insufficient exposure of the ambient light. That picture would be dark without the flash (so no motion blur is seen from the continuous dim ambient). But the flash was close, and bright, and very fast, and instantly froze the motion of the collapsing balloon and water.
The first main question will be about using Manual flash mode, or TTL flash mode? Many of us use both for different duties, and optimum would be a speedlight that can do both. But beginners with no flash experience absolutely need a flash that can do TTL for them.
And again, if you are trying to follow motion, like kids running around, or aiming at different kids at the party, you won't have much time to mess with adjusting manual flash for each picture. But once we verify TTL compensation once (in this situation), it easily and automatically adjusts for different variations in the same situation, for example like in same room. TTL becomes point&shoot flash, really handy.
We do have to sometimes tweak automatic TTL flash a bit too, but the difference is really just a matter of degree, and the initial starting point is already pretty close for TTL. Both Manual and TTL modes have advantages and disadvantages in their situations (fixed control vs. automatic point&shoot), so there are strong uses for both. Think a little, about what can actually help you. I mostly use manual flash in fixed studio situations, and TTL flash for most else (like fill flash in bright sun, or hot shoe TTL bounce flash indoors).
Terms: Note the term TTL has a few different meanings. We do say TTL generally (just meaning any automatic Through The Lens metering), but there are different designs, and for today's Nikon DSLR (those since 2003), we always specifically mean iTTL flash units (which we likely call TTL). The term TTL also specifically means older film TTL, no longer compatible. So "TTL" depends on the usage context, any general automatic Through The Lens metering, or specifically the old TTL version used with film cameras - or it may actually assume iTTL. The context generally defines it, and we get used to the confusion. :)
Broad generalities about some feature groups of camera speedlight flash models:
Flashes which can do TTL must be compatible with the camera system brand, specifically must specify iTTL for Nikon, or ETTL for Canon, etc (this compatibility is called brand dedication, dedicated to work with one camera system brand). TTL specifications must mention being dedicated to your camera brand (says "for Nikon", etc). Among other things, the hot shoe pins are different among brands.
Manual-mode-only flashes do not require brand dedication. These only use the center one pin shoe contact (same as PC sync cords and radio triggers are one pin), which provides only manual flash mode, and brand compatibility is not important then, so long as it fits into the hot shoe (Minolta and Sony use a different hot shoe, otherwise, hot shoes are standard for manual flash - or rather, standard enough for manual flash). However, if the manual flash zooms to follow the camera, it will specify some degree of brand compatibility, and communicates with camera in some degree.
Some inexpensive models only have Manual flash mode (no TTL).
Or some inexpensive models only have TTL flash mode - No Manual mode, automated flash only. Saves hardware cost of menu displays and buttons. This seems the reasonable option for those who don't want features - for automatic direct flash, just to be more power than the internal flash. But more features can really help when you get into it (zoom, and tilt and swivel and power for bounce, off-camera use, etc). Interpreting the specifications about what a flash does is not always easy, but do make sure a TTL flash is said compatible with your camera brand (Nikon or Canon, etc). Otherwise, TTL will be no go.
But today, we have several inexpensive Chinese flashes that offer full features for the same low price range. See the Review Links here at page bottom.
Flashes at B&H - Note in their left column, you can filter by Flash Type (Manual or TTL). Or you can filter by TTL System (Nikon or Canon, etc). At the top, you can sort by lowest price, best selling, or highest rating, etc. This is a powerful sort tool, and the B&H descriptions are very complete.
Flashes at Amazon - Fewer sort options, but can sort by flash brand or by lowest price, etc. Prices are good, model selection is good, but the descriptions are skimpy. Amazon does have Yongnuo and Neewer flashes, which have become rather popular, because they are inexpensive. Read online reviews. (I have Nikon speedlights, and only recently bought a Yongnuo flash, and have learned to spell and appreciate it).
Used flashes can be a good buy, but old TTL flash models for film cameras cannot do TTL on digital cameras. TTL flash is simply quite different between film and digital cameras. Nikon also had a different early digital D-TTL flash system, no longer compatible. The current Nikon DSLR do only iTTL. Canon DSLR do their ETTL. If the flash is older than 2003, there is no possibility that it can do Nikon iTTL. The only Nikon iTTL flashes are SB-300, SB-400, SB-600, SB-700, SB-800, SB-900, SB-910, and SB-R200 (the three digit models). Older Nikon flashes with only two digits, like SB-24, SB-26, or SB-80 cannot do today's iTTL. However, those old ones can still do Manual flash mode very well with current cameras (most were a fully powered class). And several may also have a non-TTL Auto mode that can meter itself, and can do automatic flash exposure that way. The old ones can't do the current CLS hot shoe communication however, so you inconveniently have to manually enter the f/stop and ISO into the flash unit menu, so Auto mode will know the camera's target goal (Manual mode does not need that, you only set power level). Nikon offers this Compatibility Guide for combinations of older and current flash and camera models, showing the features that will be available. Nikon manuals are online here.
(Too much detail, sorry) Connecting some old flashes to the camera can have special concerns. All flashes do put a voltage on the trigger contact on the flash foot or the PC connector, called sync voltage. The camera shutter shorts that voltage to trigger the flash. The ISO spec allows 24 volts, but today, this is normally around 5 or 6 volts. But some old flashes might put a couple hundred volts there, dangerous to digital circuits. All Nikon DSLR cameras are rated to withstand 250 volts (see manuals: at Optional Speedlights, or search PDF for "250 V"). The later Canon DSLR are rated 250 volts now (earlier EOS said only 6 volts). And some other cameras are low rated, so the concern about digital sync voltage damage comes up. The only way to know what you have is to measure it yourself, with flash turned on but not connected, and simply measure the DC voltage on the foot center pin to the metal foot frame (Other articles). Touching that pin to measure it can slip and cause it to flash, which is fine, that's how it works, but do expect this possibility, and aim it away from your eyes, and do not lay it face down on a surface that the flash tube heat will scorch (definitely can happen). If there is excessive sync voltage on a nice studio flash unit worth keeping, get a $50 Wein Safe-Sync to protect the camera. If is an old cheap speedlight, the $50 is much better spent towards a newer unit. All of the old Nikon speedlight flashes are a safe voltage, but old Vivatar and others, possibly not. Measure the voltage before connecting them to the camera.
Just saying though, we spend hundreds on a fancy modern DSLR, which can do impressive TTL flash things, and it seems a shame to hang on to an obsolete old flash that cannot do any of it. But manual flash can still work, and does have its own virtues.
Just a quick rough idea for beginners who have never used a camera with settings before. Wade in, a little experience will be very helpful. Speaking here of a DSLR and a hot shoe flash (which maximum power capabilities do vary of course). Note that the DSLR camera menu which sets TTL or Manual mode is only about the camera's internal flash, which is totally out of play when its door is shut. A hot shoe flash has its own menu, and totally ignores that camera menu (SB-300 and SB-400 are exceptions, they don't have their own LCD menu, so must use the camera menu, in camera models designed to recognize them).
For fill flash outdoors in bright sun, camera P mode is the place to start, and the Nikon TTL BL system can do balanced fill flash automatically. Fill flash can make a huge difference for picture of people in bright sun.
For an indoor subject, set camera A or M mode (Aperture Priority or Manual camera mode - a TTL flash is still automatic flash in camera Manual mode). Shutter speed 1/60 to 1/200 second, which doesn't matter to the flash (but see the Part 4 page). I am ruling out any camera Auto mode, which won't allow much control, and it will want to open the popup flash door, and it will use Auto ISO. Note that Auto ISO cannot be used with Manual flash (flash cannot respond to it), and it's a great idea to turn it off for TTL too. Auto ISO indoors will just make the orange incandescent lights be objectionably bright.
Indoor Direct Flash - Assuming a subject at about eight feet. ISO 100 and maybe f/8 aperture. Maybe f/5.6 for a lower powered flash, maybe f/4 for the cameras internal flash. Low ISO.
Indoor Bounce Flash - Assuming standing under a ten foot white ceiling. ISO 400 and maybe f/5 aperture (which includes a safety factor, and faster recycle, but small flashes may need ISO 800 for bounce). Don't stand too close for bounce, a normal 6 or 8 feet, but zoom in all you want. Aim flash head up at white ceiling, and pull out built-in bounce card, if present.
In all cases above, correct the results as necessary (as seen on camera rear LCD and/or histogram), adjusting Flash Compensation (TTL) or adjusting Power Level (Manual), and shoot again. If the Ready LED flashes three times immediately after the shot, that is the warning that the flash had insufficient flash power for the situation, and you should set wider aperture, and/or higher ISO, and/or move to a closer subject distance, or get a larger flash. Make it be perfect. First time will seem difficult, but a little experience makes it real easy real soon. Simply adjusting poor results as seen needed is the easy way to get perfect results.
Eight foot ceilings are relatively easy.
Ten foot ceilings need more power. These next are vague numbers with ifs and buts which depend on distance and zoom, but I'd say standing under a normal 10 foot white ceiling, Maximum level that works at (about) f/5.6 is: SB-800 ISO 200, SB-700 ISO 400, or SB-400 ISO 800. But skating on the edge at maximum power is an iffy risk for TTL automation, and instead, a little more ISO should usually be comfortable there, allowing safety factor when moving around, and faster recycle. The numbers assume you and subject are standing. If sitting on the floor with the kids, the ceilings are a lot higher.
Twelve foot commercial ceilings need all the power you can get (SB-800, ISO 400, f/4). (And a f/2.8 lens likely performs better at f/4 than a f/4 lens).
Bounce is not difficult with decent flash power, but you will need decent power for bounce. We can never have too much bounce power. High ISO helps, but when in excess, also increases seeing the orange incandescent ambient lighting too.
A metered power example in an umbrella: A SB-800 at full power (Guide Number 98, feet, 24mm, ISO 100),
mounted in one white reflected 45 inch umbrella (at 24mm FX zoom at full shaft length to fill it):
Doubling ISO adds one stop of exposure capability. Each manual half power step is one stop (for example, from 1/8 to 1/4 power is one stop more light). This power rating is not in the speedlight specifications, except the Guide Number info on next page can be useful for shopping. Charts there to compare power of Nikon brand flashes, and a method to compare any two speedlights (those sophisticated enough to have accurate specifications). Full power use causes a slow and inconvenient recycle speed however. Even half power makes a big difference (faster recycle, but aperture is one stop more open).
Guide Number may be specified in feet or meters. There are 3.28 feet in a one meter, so we simply multiply the GN value in meters by 3.28 to get the value in feet. Either way works, so long as we are consistent. Expect units of meters for other than North America. When units are not stated, small values of GN around 20 to 30+ are probably rated for distance in meters, where larger values like 70 to 100+ are rated for distances in feet (it ought to say which, but sometimes doesn't. The user manual will be more specific.) We would like to think the GN ratings are accurate, and the major manufacturers do strive for accuracy (also meaning, little variation from unit to unit). Differences we see really might be about our own measuring procedure, or about our judgment of correct exposure. There is a large section on next page about comparing power of two speedlights by Guide Number.
There are many various types of flash cables for Wired trigger, and the cable has to fit the connector. This Chinese cable and adapter stuff is inexpensive on Ebay, but Flash Zebra is one very reliable source (both reliable service and parts quality), and in particular, they provide excellent full descriptions of the parts, to know for sure which you may need. To show the connectors that I mention, I added Flash Zebra links below to typical cords showing the connector type. They have wide choices you won't find elsewhere, so buy from them, and hopefully they should forgive my links.
A TTL flash requires communication with the camera metering system, so TTL has to be on the hot shoe, or a single TTL flash can connect with a hot shoe extension cable. Otherwise a Commander capability is the only way to trigger multiple or remote iTTL flashes for digital cameras (which of course becomes a unique feature). Nikon models SB-600, SB-700, SB-800, SB-900, SB-910 and SB-R200 can work remotely with the Commander (like in two umbrellas for example). A few third party flashes (for example Yongnuo YN565EX) can work with the Commander, but this feature is relatively rare. Lower end camera models (currently D3100, D5200) do not include a commander.
If your camera model does not provide the Commander or Auto FP flash modes, then those flash features won't help (maybe on your next camera). That feature omission could make less expensive flash options be more attractive and justifiable. Nikon camera models which include the Commander also include Auto FP and FV Lock. Camera models without the Commander do not.
Opinion: In fixed situations (umbrellas, etc), I mostly use manual flash (and a hand held flash meter if multiple flash units). However, it seems obvious that indoors, everyone needs a good hot shoe TTL bounce flash (for example, to keep up with quick kids, or to do the walk-around pictures at the birthday party). And sadly, many of us need a kick in the rear to start thinking about getting better flash pictures (which bounce is). It is easy enough for anyone. For a Nikon camera, I don't necessarily imply Nikon brand flash, but my thinking is that the SB-700 is an easy choice today - especially true if you have a camera model described as "higher than low end", which can use the Wireless Commander/Remote and Auto FP HSS features the SB-700 provides. Higher price cameras like D90, D300, D7000 and up have these features too, but lower price cameras like D60 and D3100 and D5200 do not (no Commander, no Auto FP mode). The SB-700 is pricey, and its features may be overkill if you only use hot shoe direct flash aimed straight ahead (however it does also offer GN Mode, really useful for direct flash). But TTL flash mode is something essential, features you can really use, all the time. It can do walk-around TTL bounce, like at parties or to follow kids playing. It can also do manual flash mode, and has a great SU-4 slave mode that works wirelessly off-camera, like in umbrellas. The SB-700 also works as a wireless remote with the Commander (and if you have a Commander, you do want to see this feature). It should do anything you need a flash to do. I use a couple of similar but discontinued SB-800, about ten years old now, and they have been absolutely wonderful. But if you will never bother to investigate and use these better features, then a lot less flash might suit you too. But flash really is pretty easy - and never learning flash would be a real shame.