Just watched the famed Xander-centric episode The Zeppo. It's cemented my love for Xander. One of his first lines asks people if they find his spine to not step on it and by the end he shows his spine. Hey, I just made that connection!
Like everyone else who've written about it (a Google search brings up a lot of reviews), I love it because it shines the light on the goofy "useless" member of the gang and shows how brave, smart, and important s/he is. Talk about character development. He finds new strengths, drives a cool aqua-blue convertable, has sex for the first time, and saves the world without feeling the need to tell everyone, especially those who've been deliberately or off-handedly putting him down.
It takes the concept of "everyone's the hero in their own story" and really shows it.
Plus he gets shirtless. Always a plus in my book :)