Eddie Izzard, the mans a complete genius and the best standup out there IMHO.
Honourable mentions have to go to the following, who are nearly as good:
Jack Dee.
Robin Williams.
Bill Bailey.
Jerry Seinfeld.
Richard Pryor.
Bill Hicks
My Uncle Steve (not a standup but without a doubt the most pissed off old man on the face of the earth. He could out rant Dennis Leary any day.)
Just stopping by to sneak in an honorable mention for Rob Becker (“Defending the Caveman”). So funny. And so true.
He will be at the Raleigh Memorial Auditiorium, on Oct 1st. Tix are supposedly $33.50, but end up more like $40, after all the durn fees they charge. I happen to have one extra tix, btw, as hubby has decided not to go. 
Why? I dunno. But I have an extra ticket.
Already mentioned, but in no particular order:
Tony Slattery/Ryan Stiles/Colin Mochrie (I’m a big fan of the British WLIIA, but I especially love Tony)
Norm MacDonald
Steven Wright
Billy Connelly
New to the list
John Wing
I like a lot of the folks already mentioned here, but how come nobody has mentioned Bill Cosby?
And I’m showing my age here but one of the all time greats was Red Skelton. He and Sinbad(mentioned earlier) have one thing in common, they can be hysterically funny without being rude and crude.
Jay Leno used to be really good when he was a guest; before he became a host.
Steve Martin’s latest book has a hilarious chapter take-off on those commercials that warn of ALL the possible side effects of drugs.
Richard Pryor: I first saw him do a monologue on SNL about his first LSD experience. I had never even seen a drug, but he was totally funny.
Hey Baker, read twice, post once. 
OOooohhh, so I missed seeing that Cosby was already on there. Mea maxima culpa. So sue me aenea.
I second all the mentions of Eddie Izzard. Quite simply the funniest comic currently working. He mixes together observational humor (the bit about how to sing the Star Spangled Banner), deep historical commentary (“the cunning use of flags”), pop culture (“Shaggy and Scooby – the international credit card”), and pure silliness (“Je suis le President of Burundi”) into a psychedelic, head-spinning mixture. He’s also a pretty good actor; check out Velvet Goldmine and Mystery Men. Truly brilliant.
But the best stand-up comic OF ALL TIME is, to my mind, Richard Pryor. He completely revolutionized the performance form (even more than Lenny Bruce, another genius, who spent the last couple of years of his career twisting in the wind). Sure, George Carlin is smart and clever, but check out Pryor’s material, and put it into historical context; instead of just telling jokes, a la Henny Youngman and Bob Hope (and, to a lesser extent, Bruce), he digs down into his most painful, heart-wrenching childhood memories and miraculously transforms them into eye-wateringly-funny vignettes. Yes, his stories about how he was beaten as a child, or his cocaine addiction, are, as he performs them, screamingly funny – but if you stop and think about it, they must have been agonizing to live through. Pryor’s genius is knowing how to turn misery into laughter. Nobody else was doing that when Pryor came along; he not only basically invented the comic confessional, he set the high-water mark. No one else comes close.
Times like this I wish I had my own private plane. And lots of money. And no job. And no classes to go to.