I've asked Steven Spielberg and Mel Brooks questions at some Q&As, if that counts.
I've gotten pictures with Christopher Nolan, Bryan Cranston and Robin Williams, so I've done the whole "Can I take a photo with you" thing with them. Adam Sandler once complimented my accent and we talked about Europe for a little bit. I've met some big names in animation as well... Pete Docter, Tom Kenny, Matt Groening. I also read the shit out of Garfield growing up, and despite what people have to say about the comic, meeting Jim Davis was so great. My inner 10-year-old was overjoyed.
These are all pretty big names (and I'm missing a few more) and meeting/talking to these guys was amazing, but I would have to say Bryan Cranston was my best experience. Such a nice guy.