The thing with a metronome, is you want it to keep rhythm, but you don't want it to influence the music in any other way. That's why a 'tick' sound is used, it's a simple time-keeping sound as opposed to a backing track.
You will get users who prefer a certain sound for a metronome tick, some like beeps, some like hi hats, etc. Some like a different sound to signify the start of a bar(downbeat), some don't.
Any general metronome 'tick' should comprise of high frequencies, to separate the sound from the average frequency range of commonly played instruments and it's amplitude envelope should have a very short attack phase, and a short decay, to make it a precise tick.
The best thing to do to satisfy everyone, is to add features enabling the user to either select their own stored samples, record new samples or select from an in-built set of commonly used samples to use as bar and/or crotchet ticks.
Another thing to mention is the visual element, a mechanical metronome's visual pendulum swinging is a good visual cue to the player, it can help the player with expressive playing by anticipating the ticks. I don't know how detailed you want this app, but it would be a plus to visualise the pendulum swinging motion in some way.
Overall, I think as with most apps of this kind, what will make it a great app is the ability to customise it, to make it perform the way the user would like.