Love this thread.
I'm restricting myself to guys who are alive, though, or otherwise I'd just list "all of them," as I'd love to compare.
So...
Obama. As he strikes me as a smart, funny guy, and I'd dearly love to see what a sitting president is like in a fun setting with no microphones, reporters, etc., around.
Bush the Elder. I always liked him, and I admire his background. It would be a bonus if he'd say what he thought about being president...the pressures, the viciousness that is politics.
Clinton. More for the after-party. I know people who've been around him in private settings and say that the cat can flat talk and make jokes for hours.