Depends what you need. If you just need a click with tempo adjustment you can find that here. If you want something with different time signatures and accents, you can find that on most digital metronomes. If you need a visual aid some digitals also have that or you could get an old analog one. Some incorporate things like beats and are on the verge of a drum machine. Find out what you want, and find out which one has it.
uhh if it's capable of 3/4 and not just 4/4 (basically where u can put the accentuation on the beat, i.e. every 4 beats, every 3 beats etc.)

Range. e.g. 40bpm - 250bpm

Headphones component (not important)

Different types of beats (triplets, etc).

Volume (if it has lower volumes / higher ones etc)

And if it doubles as a tuner that be cool too