As much as I 'enjoy' the late Bill Hicks and Mitch Hedberg (among many others; Lenny Bruce, anyone?) I'll stick to the living, and try to stay on the positive tip. Is Emo Phillips still alive? If so, big ups to him!
Franklin Ajaye, too, was funny once (Disneyland High is both timeless and priceless). Whatever happened to him, though?
Unfortunately, I mostly only know those comedians from their CDs and vinyl records (and in the case of Lenny Bruce, reel-to-reel tapes). Fortunately for me, however, I've got a whole bunch of tapes, records and CDs from Comedy's Golden Age.
I also concur with Steven Wright; Robert Smigel (the mastermind behind Triumph, the Insult Comic Dog, and so many other, funny bits on both SNL and Late Night with Conan O'Brien); and Eddie Izzard; but would want to add both Ricky Gervais and Dame Edna to the list of the nonpareil.
And Sascha Baron Cohen! I don't care what anyone else thinks, Brüno was pure, fucking (literally), comedy gold! Does anyone know the name of the ostensibly German, gay porn star he based the look of his character on; I wonder if he got the credit he so richly deserves? Does anyone remember when Ali G asked the women at the anti-abortion rally why they wouldn't want to have an abortion? I'm not going to give it away, but check YouTube (sorry, I can't post links at the moment).
And, as I think about it, I do want to mention at least one underappreciated, dead genius, comedy filmmaker: Jacques Tati. I've never laughed harder at a film in my life than I did at his Playtime. And to think, the French awarded Jerry Lewis the Légion d'honneur! Quel sacrilege!
And while I'm on the subject of film (and also, sadly, in memorium), he was a writer on the Sid Caesar shows from the fifties (TV's Golden Age), along with Carl Reiner, Neil Simon and Woody Allen, but is most famous for having written the film and TV shows, M*A*S*H (from the also very funny book by Richard Hornberger), and is obviously one of the funniest men who ever lived, but who died just last month: Larry Gelbart.
On the negative tip: that unclever and infantile poseur, Dane Cook, bites the Carrottop! And Gallagher, he's like a giddy, unfunny George Carlin (who I don't think was all that great, either; in the end, he came off as just a stuff-shirt crank). I picture all their heads being smashed by a giant sledgehammer.
Great thread idea, Mitchymo! Maybe you could add a poll. I'll probably think of a few more names in a minute.
But I just this moment realised I don't have any women on my list! So, just under the wire, and just for starters: Sarah Silverman and Janeanne Garafolo. Dane Cook, to employ a colourful, comic expression of uncertain provenance, isn't fit to eat the corn out of either, original comic's shit! Cheers! TTFN!
NOOOOO!!! 
