I'd go with Faith too. They obviously have tons of chemistry - remember, they had their fist encounter way back in season 4. Yes, Faith was technically in Buffy's body, but how she chose to, um, maneuver it, how close she allowed herself to get, and what she said, that was all Faith - and Spike's reaction is quite telling also. The only ''problem'' could be the fact that they have such explosive personalities - one can't act like a catalyst to the other. This, I believe, could either prove phenomenal or catastrophic. But it would definitely be interesting.
Spike and Illyria - I don't know. I think their relationship could be quite intriguing, but I don't know how to picture it, mainly because I still consider her to be something like an unfinished product. I can't get a grip on her personality - will she keep switching from Fred to Illyria and back? Will Fred eventually take over completely? Will Illyria in the end be...assimilated (for lack of a better word)? I think Spike sees her as quite the challenge, and there is definitely 'something' there. I just have no idea what that something may be. Plus, the whole Wesley thing squicks me a little.
So, Faith.