I'm definitely going to go with Buffy on this one, which is sort of weird cause I never cared too much for her. When I re-watched the whole show last year, she became my new favourite and she's managed to get a hold of that position more firmly.
A close and unexpected second is Xander. He had his moments in Season 7, but before that he was kind of a jerk on some key moments. This season he seems like a different man, a better man.
Faith's arc was pretty awesome too, and again, not something I would've expected since I'm not a big Faith fan. (I certainly always liked her better on Angel than on Buffy.)
I like both Satsu and Renee. I like Renee just a little bit more, but since I think she's going to turn out to be the traitor, I try to go easy with my fascination for her. And Satsu's just too darn cute.
Like N4H I love the villains of this season. They're among the best we've seen in all of the series. They certainly stand out more than the Demon of the Week on the TV show. Twilight is very intriguing and I can't wait to find out more about him. And I had a cardboard-crush on Gigi. Too bad she had to die, cause I really would've loved to see more of her.
As for the other main-characters, I'm just not really into them, atm. Dawn I never really cared for. The Andrew-joke has been played once too often, so he's starting to get annoying. Willow has just been plain mean ever since Season 7. And Giles, what about him, he's barely been in the comics, and he hasn't had a good storyline in ages. It's not that these aren't the characters that I love to read about, it's just that right now, my focus is not on them. It might change in the future depending on the storylines.