I can't believe Sam Kinisen only has a single mention. I loved his act. I saw him twice in Houston before he made it big, once from the front row. He was unbelievable! He was just refining his early stuff, like sending trucks to Ethiopia and the necrophilia bit. The routine about going out the boys, and the wife says "Well, you won't be needing this..."
Richard Pryor. And I can't help myself, Bernie Mac just kills me. He has a book out (hardcover, too expensive) which I read cover-to-cover at Barnes and Noble. Probably a collection of his standup, he even spells the words as he would pronounce them. I was laughing so hard other patrons were glancing at me.
I know none of us ever heard his routine but just from the jokes of his I've read. Samuel L Clemens (aka Mark Twain) comes to mind.
Although I don't think he is one of the greatest, he was very good when he did it. Raw and Delirious both were funny as hell.