I'm leaning towards Willow but I can't say for sure why.

I'd have to say she's definitely more important for Buffy, just by virtue of her power (Angel's soul, the spell in "Chosen", etc.). On "to", though, I'm kinda stumped.

I think that Joss/the writers at least thought it was Xander, at least when it can to the "to". (and I will be editing my question to make the to/for distinction) In the very end, Xander was the one she trusted with Dawn. Also, when Buffy, Willow, Giles, and Xander were walking through the high school together before the fight and splitting up one by one, Xander was the last to leave her side.

when Buffy, Willow, Giles, and Xander were walking through the high school together before the fight and splitting up one by one, Xander was the last to leave her side.

True, but was that because he was most important to her, or because she was more important to him (than to the rest)?

This is a tough one. I'm gonna think on it some more.

As much as it pains me, I'm going to go with Xander. Willow turned dark and needed to re-earn Buffy's trust. Xander always had B's back, yo.

delurking in threads all over this joint

I think this is a stumper because it changes. Early on, Willow is right there, being Buffy's sympathetic ear for all of her HS drama. Xander is the goofy boy, who starts out being awkwardly attracted to her, then is awkwardly jealous and not-understanding (see: Angel).

But then as they get older, Willow's the one who goes off to college/finds a nice girl/gets on the crack!magic. Buffy, who's feeling left behind because of her isolation as the Slayer, can more easily identify with Xander's more homespun success. Plus, Xander's around, whereas Willow is raising evil temples to destroy the world. She needs Xander's quiet strength and support.

t never realized she was a closet B/X shipper until this very moment

Xander always had B's back, yo.

True, but I think Buffy took it for granted.

Well, there is that lie about Angel. That wasn't perfectly healthy, helpful and supportive, by a long shot.

No, but he didn't flay anyone, or try to kill her and her little sister.

when Buffy, Willow, Giles, and Xander were walking through the high school together before the fight and splitting up one by one, Xander was the last to leave her side.

Didn't they split off in reverse order from how she met them in WttHm? Giles, then Willow and then Xander.

True, but I think Buffy took it for granted.

I think this is true for Seasons 1-4 but not for 5-7. Xander and Buffy developed an adult friendship but it doesn't really seem like she and Willow ever did. (And by adult I don't mean porn.)