Metronomes come in all sorts of shapes, sizes, functions, and prices. For my recommendations, I'm thinking mainly of pianists, because I know that drummers like fancy metronomes as do some of the violinists and wind players I've worked with.
The first thing you need to ask yourself is, what do you think you'll need this metronome for?
Do you need it to keep a steady beat?
Do you want general tempos or exact beats-per-minute?
Do you want it to mark the downbeat?
Does it matter how many sounds should makes?
Does it need to keep track of complex time signatures?
Do you want it to perform cross rhythms?
If you answered either "no" or "I don't know what that means" to the last two questions breathe easy, because you're about to save a bunch of money. Really high end expensive metronomes perform a bunch of functions that most amateur (and professional) musicians have no need for. In my experience, I prefer to handle these kinds of things on my own anyway, rather than leave it to a metronome.
I prefer to keep things as simple as possible. Two things matter to me overall:
Is it easy to operate?
Can I stand the tone?
I'll tackle the second question first. You're going to be hearing that click or beep a lot as you practice. Think about the average piece: four beats in a measure, anywhere between 16 and 200 measures in a piece and you may practice the piece for an hour. In one practice session you have the opportunity to hear the metronome sound thousands of times. It better be a sound you like. Take time in the store listening to the different sounds. I've bought expensive and expensive machine before only to never use it because I hated how it sounded. I really like the classic sound the most. A super digital beep can get on my nerves but I find the analog, old-school click is satisfying and not too distracting. My favorite metronome (which I'll profile later) has a great sound despite being closer to a "digital" style.
I value simplicity in my metronomes. If I have to choose between multiple settings every time I turn it on it's no good for me. I want to be able to turn the thing on, set it, and start practicing without any extra steps.
If you want the classic feeling and have some space for it, get an old fashioned metronome. I think they have the best sound for practicing with piano. The power comes from twisting a small crank on the side, similar to winding an old wrist watch, so you don't have to worry about batteries. The downside to this is that it adds an extra step before you can practice and the "power" can die before the end of your session. Also, it won't turn off immediately, it will just slow down, so you can find that you're slowing down as you practicing. The tempo is determined by moving a slider up and down a rod. It's inaccurate but I actually really like that aspect of it, but for sort of philosophical and musically spiritual reasons.
If you're looking for a digital (technically a "quartz") metronome I strongly recommend the kind that I use:
|The Matrix MR-500|
I like the tone of it, plus the dial interface has the slight inaccuracy of the analog metronome but still with the precision of a digital machine. You can only choose settings by multiples of two and (in the higher tempos) four from 40 to 208. There's also a 440Hz A for you to tune to. When I was a kid I was terrified of that tuning noise for some reason.
This metronome is great for a number of reasons. It's really simple to operate. It's pretty light but extremely durable. I like the sound of it, it has a headphone jack (which I've never actually used) and there aren't any menus or screens to flip through. Plus I think I've only replaced the battery (a single 9-volt) two or three times in the 15+ years I've used it.
There are similar models in a "credit-card" design which is flat and uses buttons instead of a knob which I like too. The style I try to avoid are the ones that will mark the time signature for you. If you choose a 4/4 time signature it will give a different, louder noise on beat one of each measure and softer tones for the other three beats. They're difficult to use because you're constantly waiting for the downbeat to come around again, and if you have complex or changing time signatures they become almost useless.
So if you're going out to get a metronome, get something simple, inexpensive, and somewhat pleasing to the ear. If you want to hear what different metronomes sound like, visit the website Metronomes.net