Willow, Tara, Buffy, Write-in Candidate.
Willow has always been my favorite. Like Xander, I love crayon-breaky Willow and I love scary veiny Willow, and all the stages between and after. She has issues, but that makes her a complex, conflicted character with a depth of personality you really don't often see on television (or anywhere else, for that matter, especially in these days when formula writing is pretty much the standard...) I liked her trial-and-error approach to figuring out her own style when she went to college and started picking out her own clothes (and boy, were there ever some errors!) And I like that she's basically a good person, who, when finally faced with her own bad side, didn't like what she saw.
Tara. Sweet, thoughtful, intelligent, a strong sense of ethics, and beautiful. What's not to like? Oh, yeah, she's a lesbian. Well, there was an old joke long before Riley got in on the act that went... um, never mind; let's just say "I'm a lesbian, too." Okay, so I'm a guy, big deal...
Buffy. From a purely physical standpoint, she was "hotter" pre-Season Six. But she has a strong sense of duty, she's loyal to her friends... oh, yeah, and did I mention - stamina?
And here's my write-in candidate: Kennedy. She's another looker - does Willow know how to pick'em, or what? (Okay, so technically, Tara and Kennedy both made the first moves on Will, picky, picky...) It's pretty obvious from the way she's stood by Willow that she'd walk through fire for someone she cares for. And, like Buffy, she has a strong sense of duty: when Faith's team was caught in the tunnel blast, it was Kennedy who pulled the survivors together and damn near got them out - along with Faith, whom she could have abandoned, and none of the others would have blamed her, because they were ready to do just that. And when the Turok-Han attacked, she's the only one who went on the offensive and tried to protect the others. Tough chick. One thing's for sure, you'd always know where you stood with Kennedy. We'd probably fight a lot, but the making up would be fun. Except that she's a lesbian, too. Dammit, Willow gets all the best stuff...