That's a tough question. . . I imagine Midna from Twilight Princess would be really fun to hang out with, Liara T'soni from the Mass Effect series also seems like someone I could get along with quite well (since she's the bookish type). Tali from the Mass Effect series is also someone I would like to befriend, the only problem would be finding food she could eat and the need to be very, very careful to make sure she didn't get a tear in her suit. Actually, the lack of medical knowledge for healing all of these people would be a problem if they were injured.
I also think Garrus would be a cool friend to have, and I would also like to see how Mordin would handle various social situations (I imagine his singing would be a hit at parties). Joker from Mass Effect would also be fun to hang out with since he's pretty funny. I also wonder how a friendship with Legion would work, or if it even could in today's society.
Gee, I'm beginning to see a pattern here, no wonder the Mass Effect series is one of my top favorite game series ever. Most of its characters are people I would actually hang out with.
I would also like to hang out with Samus Aran, though I should state that I have never played Other M and as such, I am unaware of her characterization from that game.
I wonder how a friendship with The Boss would work, but I don't think I'm the kind of person she could be friends with. It would be more of an admiration thing really.
Though, if I had to pick only one, it would be Liara. I have a feeling we would have many similar hobbies.