There's a difference between being "derivative" and flat out joke stealing. Most comics are the former, Denis Leary and Robin Williams are the latter.
Don't forget Dane Cook.
Robin Williams has an undeniably hilariously funny shtick onstage, and I sometimes sincerely believe he's improvising more or less his entire routine. I'd suspect a lot of the jokes he steals are because he's so addled or loony that he really has absolutely no idea where he's heard the joke before or whether he wrote it or not. They just pop in his head suddenly and, Robin Williams being Robin Williams, he says it without thinking.
My own nominee for funniest person alive is Fred Willard.