Real: Don't hate me for it, but I'd probably choose Adolf Hitler, or someone of his level of fame.
I'd like to find his true character, and know what part he has to play... would be enlightening.
Fictional: Probably someone like the Emperor of Mankind (40K), so I can bug the hell out of him, and learn a load of stuff (So what if my mind breaks, it'll be worth it). Plus I could tell him he is a moron for what he did on Nikea. And spoil his future :3 (Not like he didn't already know... he is a schemer on a whole other level from myself >.>)