I find Xander really excellent, mainly because his storyline is linked directly to two of the main values of the show: Feminism and recognize the worth of one self.
You see, Xander was a conditioned teenage boy who’s been brought up to accept and live up to the gender roles imposed by society. The way he saw and treated women in the early seasons isn’t really empowering and his behavior betrayed many traits of a Nice Guy TM. But the Buffyverse doesn’t only show you strong women to illustrate their feminist message, they also make it shine through the acceptance by the male characters. So Xander was dragged into this new world of demons in which girls are superheroes. And Xander’s not just the guy who is normal. He’s the guy who felt like he was worthless all of his life, because his family told him so and grows up to see his friends get powerful and YET comes to terms with the fact that he is not. He stops seeing the line between men and women and looks up to his friends simply as heroes. Xander, no matter how unwise some of his decisions were, was thinking with his heart, not his male parts. And as he gets older, he finds something he likes to do and is good at and that’s enough for him. What matters is that he does his part to do what’s right and protect his friends. He finds out he has something to bring to the group because he has a lot of heart and courage.