My favorite character is Giles. (Who saw THAT coming?) Reasons:
Anthony Stewart Head is smoking hot and has the loveliest voice, both for speaking and for singing.
Giles is kind, intelligent, bookish, slow to anger, and has a good sense of humor -- all qualities I admire in a person.
Giles *always* puts Buffy and/or the mission above his own self-interest. He has amazing courage. He risks his life without a second thought, and soldiers on through grievous injuries.
Giles' fatherly tenderness toward Buffy is so endearing that it just takes my breath away. I also like his fatherliness toward all of the kids. I like his role as "stalwart standing fast" -- always supportive, always on the side of his kids.
But he can be a BAMF when needed, as he was with Travers' minion in Checkpoint and with Glory's minion in ... the ep where Tara was "taken" and with Ethan Rayne in several eps and with the flaming baseball bat attack on Angelus.
He can also be a bit geeky, especially in the first two seasons, but that's charming too.
He has the ability to see the good in people despite their flaws, like with Faith.
He's not flawless, which is also appealing. But he seems self-aware and understands what his mistakes and shortcomings are.
He has a darkness to him that he has struggled to overcome - Ripper; and the Band Candy persona; and the dark side probably emerged when he killed Ben so ruthlessly.
I will probably think of more later.