Some great ones above. A few geniuses that also come to mind:
Lewis Black
Chevy Chase
Mitch Hedberg
Chris Rock
Carlos Mencia
These perfected their art for only about 15 minutes, but when on top, there were few funnier:
Rosanne Barr - early stand-up was stellar
Dice Clay
Bobcat Goldwaithe
Whoopie Goldberg - yes, she was actually funny once upon a time
-dave