I'm with Rob here. This is just too tough to decide, and really changes quite a lot for me. I always find the newest OT figure (especially if it's a background character) that I get my mitts on to be my favorite figure of the moment, but I suppose if I absolutely HAD to choose one, and only one, it might be the 300th Boba Fett. Followed closely by the Vintage Fett (nostalgia), RH Vader, Commtech R2 & Stormie, and many, many others!
But if I was actually stranded on an island, I'd probably go with the Commtech R2-D2, since he seems like he'd be a nice, friendly, supportive, good listening kind of pal that I'd like to have around. Kinda like that volleyball in the Tom Hanks Castaway movie a couple years ago, ya' know?