Hmm, it's actually a tough one. There are lots of people I'd love to talk to and picking a single one out of the lot is more difficult than it should be.
Off the top of my head though, I'd probably say Hitler, as morbid as it sounds. I think it would be really fascinating to see what exactly it takes to drive a man such as him to not only commit some of the most heinous crimes against humanity, but to successfully convince so many of his fellow countrymen to do the same. I think it would be very interesting to hear what he would say in his own defense, or even if he would defend what he did. I've always been fascinated by the psychological state of history's biggest villains. It's hard for me to imagine hurting one person I don't even know, much less doing it to millions.
If I had to choose someone good (and I know I'm breaking the rules by giving two), it would probably be the Dalai Lama. He's just such an amazingly kind and tolerant person.