Spike or Buffy. I think between Buffy and I we'd find a way off the island for sure, and choosing Buffy is smart as well because her friends always end up looking for her eventually, and if it was Spike and we really couldn't find a way off, I'd just get him to turn me vamp *shrugs* and we'd just sleep the day and hang out having sex all night, and stalking wild animals, and bitching about having no movies or tv.
I wanted to say Giles but I'd be worried I'd seduce him, and he didn't have much luck with the ladies either it seemed...
I also wanted to say Xander because of being a carpenter and we could build shelter at least. Or maybe Willow, I would probably just have to keep lending my energy for new magic to try... although that is really sexy... 😉