I have a thing for Ms. Carpenter first of all, and She just was funny and brought alot of comic relief in Buffy's dark times. I liked when she went to Angel but I wasnt too big on what they did with her character.
Of Course I like Willow and Buffy as well. They both brought depth to the show. Very good times.
Tara had a vulnerability that she was able to overcome, and pushed herself to face her fears. I really liked that.
Anya was a bit Cordy-like in that she could bring humour to sticky situations. Unlike Cordy she was very much a stranger in a strange land, and was able to make some very pertinent observations about the human behaviour she was immersed in.
You know, I've never actually thought about it. They are all such rich characters that it makes it really hard to choose. I guess if I had to pick one, it would be Xander in all of his unperfect glory. I love how he developed but stayed the same backbone (I know that sounds redundant) of the series.
XANDER!!! Xander,all the way. Though closely followed by Willow. Xander is such a regular guy. He's just Xander. All loveable, and loyal, and trying to do the right thing. Plus, he's one of the few leading type guys who has trouble getting the girl. If you ask me he had the most character development.
Having said that, I also adore Willow. I don't think Xander could have been what he was without willow.