George Carlin and Pryor were the probably a couple of the greatest of all time. Usually they are regarded as the top 2. Eddie Murphy was fantastic with Raw, but he got too big for his own good and lost touch with reality. I always loved Bill Cosby... definitely a legend, but he's too old at this point (he still tours though and I'd gladly go see him). As of active current comedians... Dane Cook gets flack, because of how popular he is, but you can't deny the guy is funny. Eddie Izzard is fantastic, but is another underrated comedian who became overrated.
I have a lot of friends who are small time stand-up comedians. There is an absurd amount of people doing it at this point. Many are hilarious, but you probably will never hear of them. I really can't think of any "superstar" stand-ups right now. It's basically Chris Rock and Dane Cook, I guess. And then whoever you see on Comedy Central specials and "Last Comic Standing". I mean guys from Def Comedy Jam and such are funny, but I don't regard any of them as "best". Kat Williams, for example, might be funny ... but meh.
As for Robin Williams. Some people here are saying he's overrated. Could be. Williams was great back in the 70s though. His entire act revolved around doing lots of cocaine and his manic personality was an admitted result. He was pretty funny back then. He's still kinda funny. But from speaking to my friends who do stand-up, they tell me that Robin Williams is notorious for being known as the guy who will cut checks for jokes... or just flat out steal material. He has told stories of comedians who will completely change their routine mid-act the second Robin Williams walks in, because they don't want their material robbed. Kinda interesting.
Edit: I guess Jerry Seinfeld still tours sometimes. I guess you gotta at least mention him since he probably makes more money doing stand-up than any of the above.
