Hmmm, a fictional character?
Just kidding (sort of).
Actually, this choice is pretty easy for me, I’d pick this guy:
No, I don’t mean Henry Cavill, the actor (although I wouldn’t mind getting to know him, too), I mean the comic-book character, SUPERMAN!
As for WHY? I’d assume that answer is pretty darn obvious. Why wouldn’t you want a best friend who is the most powerful being on the planet? Seriously, DUH.
As for what it would be like, we can just look to the man whose been portrayed as his best friend for decades in the comics. SUPERMAN’S PAL, JIMMY OLSEN. Jimmy was always having crazy adventures, traveling through space to other planets, time traveling to the past or future, encountering aliens, monsters, and such. Virtually anything could, and often did, happen in a Jimmy Olsen comic.
So you get to have tons of exciting adventures and, no matter dangerous things may seem to get, you’re never really in any danger because Superman will always show up to rescue you at the last minute.
So that’s my answer.