Well you already admitted to Jack, and Rebecca seems the obvious third option. [You need more friends Spike. As for that question he actually has to think for a moment. It is an interesting one.] Jack, obviously. Out of curiosity mostly, and because he's so obviously enjoy himself. I don't think there's anyone else I'd like to see for sexual reasons, just to see what they'd look like. Martha, maybe? Or Kitty? [Alright, so clearly he needs more friends, too.]