Most of my list is based on "what could they do for me" rather than just "I wonder what they're like in real life".
1) President of the United States
I don't care who it is or from what party, having the president as your best friend has got to have some awesome perks. Late for work? "Sorry, I had a meeting. You know, with the president." Get a parking ticket? "Do you know who my best friend is?" There'd be plenty of opportunity to drop suggestions you'd want to see done, and even if he didn't end up following most of them that's still a considerable influence over the country. The big downside would be that the opposing party would try to publicly demonize you, but I could live with that (I might even get a kick out of it).
2) Chris Metzen (of Blizzard)
I don't know how much he'd be considered a "celebrity", but he is one of the most known people at Blizzard. Ever since the original Warcraft, I've spent A LOT of my time playing Blizzard games. Metzen is responsible for most all the lore behind StarCraft and Warcraft, it'd just be fun to have that kind of connection. Beta access to new games? Try pre-beta friends & family access. Then to top it off, you'd have a direct line to the higher ups at Blizzard for getting your suggestions into their games.
3) Kevin Smith
Kind of an obvious choice, but I think it'd be genuinely entertaining hanging out with him & his circle of friends. This is the one entry on my list that isn't so much "what can they get me", but rather he seems like the kind of person that I'd want to be friends with.
4) Member of my favorite music group
I'm not even sure who my favorite band/group is now days, but it's kind of irrelevant to the list. Being best friends with one of them would be a blast. I could go on tour with them for free. I'd get to hear new albums before they were released, and all the non-released stuff that never makes it public. My only concern would be if knowing them personally would ruin their music for me, but I don't think so (it might depend on the group). If they were a big enough band, there'd most likely be lots of additional perks as well.
5) Michael Bay, circa 2004
That way, when he got hired on to do Transformers and approached me saying "I had this great idea to put flames on Optimus!" I could slap across the face and a make a positive change for the future.