Frank Caliendo has an awesome John Madden Impression.
He has a bit in rotation on XM Comedy 150 where he bounces between a bunch of impressions...Madden, Clinton, Bush, Kasey Kasem,a bunch more. It's friggen great.
I also like Doug Stanhope, but he definitely has more of a cult like following that is not for most people.
One guy who I used to hate but now like is John Leguzamo (sp). I think he is on ER now. I think he did an HBO special that they play bits from.
Rodney Carrington has some great comedy songs (I think I'm dancin with a man was one I heard today)
Ron White is pretty decent, but overexposed thanks to Blue Collar.
Dane Cook has some good stuff, high energy which I assume would make for a great live show if you could find him in a smaller venue (good luck)
Chapelle and Murphy are friggen classics. I like about everything I've heard from them in their primes.
There are more that I can't think of off the top of my head. XM channel 150 really introduced me to alot of great performers (or at least great bits).