I am going to assume this is open to things like fictional characters, celebrities, and the dead; otherwise why would this question even exist?
I generally and vastly prefer to be the GM, so this isn't a question that brings great excitement or interest in me. That said, I'm going to say Seth MacFarlane.
Given what I've seen from The Orville (my current favorite Science Fiction anything) I think a Science Fiction campaign run by him would be right up my alley. I have no idea if he plays RPGs or ever has but if he was into it and gamemastered I'd really like to give a MacFarlane run game a go.