Eddie Murphy or Bill Cosby. Actually it might be Eddie Murphy's impression of Bill Cosby. I think Cosby brought so much to stand up comedy and every single part of his bits are funny, all of his mannerisms really accentuate the stories involved. Eddie Murphy did a few things in Raw that I didn't think were to good but Delirious is the best stand up act I've ever seen. Those two are followed closely by Kenvin James (Sweat the small stuff, he's in the King of Queens) and then many others who are great but aren't my favorites:

Lewis Black
Jeremy Hotz
Pablo Fransisco
Zach Galafinakis
Dave Chapelle
Dane Cook
Rodney Carrington
Richard Pryor
And others that escape me at the moment.