Great answers folks! (Extra portion of garlic bread to Nichole's table for the excellent answer to the "why" part).
It's easy to think of some, hard to whittle that down to just one!

Einstein would be interesting. At the tail end of his life, he said that his "universal constant", which was a deliberate fudge to try and make reality conform with his vision, was the worst mistake of his life. I'd love to see his face when we talk about the current "dark energy" conundrum!
My shortlist would probably be Chalmers, Kant, Feynman, Ramachandran or Wittgenstein.
Chalmers for a good dualist vs materialist argument over consciousness.
Kant for his brilliant insight into straddling empiricism and realism.
Feyman because his work on quantum electrodynamics was a total stroke of genius and I'd love to know how that popped into his head.
Ramachandran for his amazing work on the mind & brain (would also have the consciousness conversation with him too). His 2003 Reith lectures and book "Phantoms in the Brain" were inspired.
Wittgenstein...well, it ties in because of his work on metaphysics, the existence and properties of entities without observers or experience, etc..
Choice: probably Chalmers or Ramachandran, as I'm nowhere near bright enough to keep up with any of the others. They'd get bored talking down to my level. (Sorry Steve/Rama)