Buffy, for all of the reasons already stated by Joyce Summers.
Mostly, though, I want the super strength and agility and reflexes and other cool stuff. I am all lacking in strength. I'm clumsy and I fall down a lot and pass out sometimes and God forbid I should attempt to walk in a straight line. lol. So, for me, being in a super cool, high powered, a zillion-times-better-than-average body would be awesome.
Barring that, I might try being one of the vampires for a night. I mean, that has to be pretty neat, the whole being dead yet walking and being part demon thing. Must feel weird, but neat. I'd try it for a day. No real gender preference on that one. I think being undead would be enough of a change that gender would be pretty far down on my list of things to experiment with.
Oh, and I feel obligated to say that I'd be Joyce in Band Candy. Hmm... maybe that should be at the top.