I would have to say Willow. I think for me, she'd be one of those friends who is just enough like you and just enough different that you can't help but be friends with them. Ideally, though, I think we'd be a little more than friends. 🙄
Anya as well, because I find her funny and loyal. I don't think I could be friends with Buffy, she's nice and all but a little bit too serious and sarcastic for my tastes. Xander, maybe. Tara, probably but she wouldn't be one of my major close friends. Cordelia, never.