Tastes change, as do the times. My current favourite is Chris Rock. Years ago I saw Gary Shandling live at Roy Thomson Hall, and that was uncontrollable side-splitting laughter right through the whole show, so just based on that, I would have to rank him at the top. Robin Williams can be brilliant and funny. For ad libbing, he would be hard to beat. Incidently, his comedic talent is about equal to his talent as an actor in films Good Morning Vietnam, Dead Poets Society, and a half dozen other titles. Going back to the 70s, Cosby was amazing, but I find his more recent comedy not nearly as funny. But all time great stand up comedian would have to be George Carlin IMHO. He been around forever and his act has cleverly evolved. Great social commentary, rather than empty fluff, which of course can be funny in its own right.