Definitely Spike. I actually think Angel and Riley are closer to my usual type, since I usually like broad-shouldered, well-muscled guys, but Spike just...well, I can't really express it on a family-friendly board. Maybe it's the combination of my love for nice cheekbones, strong jaws, light eyes, and deep voices that does it, but hot damn, that man is beautiful!
Wesley, Doyle, Giles, Lindsey, Gunn, and Riley (hey, as much as a lot of people may hate him, you've gotta admit he's not bad-looking! Besides, I don't hate Riley nearly as much as most people do) are all up there as well. I'd also include Angel and Xander in that list of runners-up had they not gotten pudgier as their respective shows went on...I stopped finding Angel attractive after about Angel season 3 and Xander around Buffy season 6. I was going to include Oz as well, but then I realized that I was basing this purely on aesthetics and that Oz's hotness was mostly derived from the awesomeness of Seth Green.