Shouldn't this be a poll? Well there are only a few people that can even be considered in the running:
Bill Cosby (good clean wholesome family fun...and Jello)
Richard Pryor (the anti-Cosby)
Eddie Murphy (Richard Pryor, the next generation)
Chris Rock (Eddie Murphy, the nexter generation)
Dave Chappelle (I'm Rick James bitch)
Now those first three haven't done much (I am ignoring most of Eddie Murphy's movies for the past ten years) for a long long time, but I'm still laughing at their classic shit. So greatest of all times (at this point) I'd prolly have to say Pryor, even though I haven't heard as much of his shit compared to the others. However, funniest one today I think I'd have to go with Chappelle.
edit: The poll went up while I was making this post. Too lazy to edit the post now, so just read it bitch