Impersonations I do fairly well, based on audience ratings:
Bill Clinton
Morgan Freeman
Assorted characters from The Simpsons
Randy Savage
Hulk Hogan
Diedrich Bader
Christian Bale's Batman
Those are all that come to mind at the moment, I get a lot of practice at work with patients when I try to make them laugh. I'm currently working on Phil Hartman and Mark Hamill's Joker, but those are fucking hard.
Anyways, people always seem to think that Nicolas Cage is easy to impersonate, maybe it's because it's not as much his voice as it is just changing your pitch and screaming every other word.