They all sound somewhat different. I have a Digitech one that just sounds poor as a wah. I also have a beat-up King Vox wah from 1973 or so that sounds great except for a horrible noise right at the point of clicking the wah on/off switch (at the end of the pedal's throw) and generally scratchy noisiness. I recently bought a Budda Bud-Wa which sounds very close to the Vox but is better-made; its other advantage is that the bypass is hardwired so if you are nor using the wah, the guiotar signal does not go through any of the pedal's circuitry, it goes straight to the out jack, so there's no "tone sucking" going on when the wah is not in use.
Best thing is to try all that you can get your hands on and see what you like. The Cry Babys seem to be quite popular.