Let's see... in no particular order:
1. SMS- I would love to play or watch him GM a game of Talislanta.
2. Sandy Peterson- I would love to play or watch him GM a game of RuneQuest (II or III).
3. E. Gary Gygax- Come on, who wouldn't want the opportunity to play or watch him GM a game?
4. Dave Arneson- I would like to see the contrast in styles between the two founding fathers of RPGs.
Wow, coming up with only four choices was difficult! If I could sneak in one more, then it would probably be Steve Jackson. I would love to see him run GURPS seemlessly off the top of his head.