Not even any contest. And I promise it's for reasons other than just how incredibly attractive I find him. And...this video:
Giles is such a wonderful example of how it is possible to be paternal without being paternalistic. He was always there to teacha n guide Buffy, and while he pushed her into fulfilling her potential he always tried to let her make her own decisions. In fact, in the sixth season he realizes that Buffy needs to learn to do things on her own so that she can grow and he leaves so that she can develop skills without leaning on him.
He also has an interesting past as a crazy troublemaking demon-worshipper which he eventually overcame. When it resurfaces as an issue in "The Dark Age" he tries his best to handle it himself and keep his watchee out of it. Also in "Band Candy" he listens to Cream and smokes cigarettes and does Joyce on a police car. Twice. Swoon.
Giles is not only a father figure for Buffy, either--he serves as a paternal force for all of the Scoobies. Think about it. Buffy's dad is practically out of the picture for the whole series, Xander's parents are often referenced as being scary and dysfunctional, and we never hear about Willow's father at all. As far as even the secondary Scoobies, Anya has no parents, Oz's are never discussed and Tara's is crazy and chauvinistic in his one appearance. Giles has an especially tight relationship with Willow. He takes her back to England with him after she recovers from her dark magic overload so that she can recover. He also knows how to be tough with her when need be.
The amazing thing is Giles' willingness to break with the Watcher's Council, the patriarchal organization discussed in my post on Season 3. In "Helpless" he objeccts to the test and then makes it invalid, saying he'll do whatever need be done to stop it. After this he remains Buffy's Watcher in a more personal capacity even after she herself breaks from the Council.
And he IS a personal figure for her. He is the first person Buffy calls after she finds her mother dead in "The Body", and he reacts in an almost equally shocked and frantic way as Buffy herself did. HE clearly grows to love Buffy and her entire gang.
His position also forces him to sacrifice his own personal life, to a degree. He loves Jenny calendar but must face her death as maturely as possible. He is tortured more than once in his capacity as Watcher. Even his relationship with Olivia is interfered with--she visits him during the episode "Hush" and sees The Gentlemen come past his window. But his devotion never, ever wavers.
Giles never let me down once through the entire series. He always has good one-liners, causes some of the biggest tearjerkers of the whole show and makes all of the other characters better. He is the Mr. Feeney of Buffy The Vampire Slayer, and then some. And that is why he is my favorite.