1. Willow: Her characterization definitely suffered in S6-7 but she remains one of my favorite characters of all time. No other character can cheer me up with gleeful, bubbly joy and then turn around break my heart with her angst. Plus, I just adore what a smart self-starter she is. She learns how to hack and decrypt discs, do magic, program robots and become a robot with little to no guidance- just pure Will-Power.
2. Xander: He puts aside all of the bullshit from soulmatey love to power tripping to focus on being the best he can be in life and the fight against evil, all with no powers. That means a ton to me.
3. Buffy: The martyr and vampire complex seen since S2 puts Buffy below her best friends but other than that, Buffy rocks. I adore the fierce warrior, petulant teen, valley girl, etc. At her best, she's possibly the most appealing Epic Hero ever because she brings the concept down the earth and makes it an emotional experience like no other.
4. Giles: He'd be higher if he was a little less Buffy-centered and focused more on mentoring and befriending Willow/Xander/Faith but I still think he's incredibly special. He's amazing as a repository of knowledge and wisdom and a dispenser of some of the most elegant British snark ever seen on TV.
I'm a Core Four-centered viewer so they're by far my faves. Although, I give honorable mentions to Cordelia and Oz. You make think that Cordy and Oz were very different but they did have stuff in common- being hilarious, looking attractive, having a colorful, if shallow, outer life that it brought dimension to the insular Scooby gang and in their relationships with Xander and Willow, having the two most equal-in-power relationships on the show. Which is great because the power disparity in the other relationships could be interesting or just plain annoying but often made me uncomfortable for a supposed feminist show.