This is by no means an exhaustive list.
Historical
Jesus of Nazareth: I think the reasons would be fairly obvious.
Mary, Mother of Jesus: Because I worship the bishop of Rome. Duh. More seriously speaking, Luke is the only source in the gospels which provides an account of the Virginal Conception and Jesus's childhood. To know what Mary had witnessed before, during, and after her Son's ministry would be one of the most intimate explorations of the life of Jesus.
St. Paul the Apostle: I would have more than a few questions about his theology (e.g., on grace, faith, etc.). I also admire Paul's tenacity, resolve, and, of course, his beautiful letters. I would be so eager to ask him about certain passages from his aforementioned letters, such as Philippians 2:6-11.
St. Augustine of Hippo: He is a giant, if not the giant, of Western Christian thought. It would be like Twilight Sparkle meeting Star Swirl the Bearded---not that I am the prized student of Princess Celestia.
Thomas Merton: I would have liked to have known his response to more recent conflicts (e.g., the Iraq War) through the prism of Christian peacemaking.
Alexander Hamilton: Instrumental in the formation of the American political and financial systems. How would his vision of federalism mesh with the current state of the United States?
Abraham Lincoln: Perhaps more conventional but no less meaningful. Beyond picking his brain over the Civil War, I understand that Lincoln had a sharp wit which his contemporaries either overlooked or considered inappropriate.
Franklin D. Roosevelt: The longest serving American president who was instrumental in shaping America's domestic and foreign policy for the next half century. Could he have ever imagined when he first took office that he would leave such an enormous mark on US (and global) politics?
Raymond Brown: He was a preeminent Catholic biblical scholar (my father knew him personally). I would have enjoyed talking shop with a man who was highly regarded in his field.
Mel Blanc: The man of a thousand voices. I'd like to hear from the one that gave birth to the other 999.
Philip "Lefty" Weinert: My grandfather, with whom I never met personally. To get to know him, to talk baseball with him---that is pretty hard to beat.
Contemporary
Barack Obama: I have the impression there is a lot more going on in his mind than people may give him credit for. It seems to me that he would make for an interesting a thought provoking conversation.
"Weird Al" Yankovic: I've met with him briefly, confined to autographs after his concerts. From stories about him and from personal digest, my sense is that he is a smart, thoughtful man who also happens to be arguably the best satirist of this generation.
Charlize Theron: So I could drool on the table while sitting across from her. Way to make an impression!
Lauren Faust: And then we get serious again. I would want to thank her profusely and praise her work, undoubtedly causing Mrs. Faust a lot of good humored discomfort.
Tara Strong: She always comes off as a down to earth and genuine person with gobs of talent to boot. I think those qualities alone speak for themselves.
Tabitha St. Germain: I listened to her interview on Everfree Radio. I came away infinitely impressed. Tabitha would likely be the cast member I would talk to first if the conversation could not revolve around ponies.
Gary Cohen: Because every Mets fan has fallen in love with the sound of his voice, through good times and (mostly) bad. And the two of us could talk baseball---lots of it.
That's enough for now, I think.