Given the choice of anyone in the world, whom would you want as a dinner guest?
Nikola Tesla. It isn't a very original answer but the challenge isn't to be original, just authentic, and Tesla is one of the most fascinating people I've read about. In addition to probing his mind about everything even slightly cryptic he's said, as well as to clarify some things like "did you really say that to Marconi?", I'd like to get his take on modern technology and what he'd do different.
I'd also like to ask him about the odd event early in his life where he had some near-death experience, after which is apparently when the genius really opened up inside of him. I'd want to see how much truth to that there is, and what he feels about it.