I'd have to say Chris Rock is the best comedian out there bar none.
Not only does he make you laugh...but he makes you think.
He's achieved a certain comedic status that only a few others have achieved...that level where they are treated like rock stars.
You had Richard Pryor, then Eddie Murphy...and now Chris Rock has picked up the torch (of course lit from Richard's hair

) and is carrying it quite well.
There are a bunch of others out there that I truly enjoy watching:
D.L. Hugley
Dave Chapelle and if you haven't caught his new show on the comedy channel I suggest you watch it.
Bernie Mac
just a few of my favorites.