It really depends. Different people have different interests, strengths, etc. So I don’t think I’d limit it to just one person. Also, while there are tons of people I’d like to meet, Fluther is an online forum where good writing/structure in one’s answer is crucial, so that excludes quite a few historical personages (some of whom may be fascinating in themselves, but not as articulate as we might like them to be!). That said, here’s my answer.
Like others here, I’d be interested in what Thomas Jefferson, Mark Twain, and Oscar Wilde have to say on a variety of topics.
Others I’d submit for membership here include Voltaire, Thomas Paine, Ann Landers, Teddy Roosevelt, Molly Ivins, Dorothy Parker, Simone de Beauvoir, and Sir Richard Burton (the explorer). I’m sure I could come up with more, but that’s a good start for now.