S3 had two things going for it - 1) the best production values, next to maybe S4 and S5, for some reason the visual quality is better for most of the episodes. And 2) a cohesive plot that involved all the characters and didn't focus on romance. Romantic plots tend to be focus only on one or two characters. S3 and S5 focused on the slayer mythology - which is another reason...they tend to be favorites with people who aren't shipping a specific character. Add to that - if you were a Xander and fan, S1, S2,S3 and S4 were his best seasons.