Hey,
Hmmmm...I've been listening to a lot of newer rap recently, it doesn't seem like a lot of it has any reverb in it at all, rather there is a lot of double-tracking and clever editing.
Since the point of a rap music is to bring the vocals right to the front of the track, it would be best not to use any reverb at all, reverb will tend to make the vocals appear as if they are in the back or distant. Also, because of the nature of reverb, it will tend to muddy the vocals up and cover all the other sounds in the background and across the stereo field.
A good effect, though, is to run your vocals through a pitch shifter and mix it back with the original. This is how Tu-Pac got his sound (I think), this can make the vocals appear tough and thick without losing clarity that reverb can do.
Nick
BTW If you're looking for 'clean' reverbs, and big favorite is the TC Native Reverb...