Which actor would you want to meet?
I'd want to meet Richard Harris, the original Dumbledore. Much to my everlasting dismay, he passed away in 2002. I like Michael Gambon just fine, but I loved Richard Harris. Plus, he was King Arthur in Camelot, a musical that played an integral role in my childhood. (Because, you know, movies about marital problems and adultery and betrayal should be the basis for every little girl's growing-up years.)
Also, the dude was a STUD when he was younger. Consider the following...
Runner-up: Emma Thompson, which is sort of cheating because I wouldn't bother with Harry Potter talk. I'd want to chat about every other awesome movie she's made. That woman is the bomb-diggity.