I'm pretty partial to the first three seasons. They keep it simple in the high school. You get Cordelia joining the group and the introduction of Anya and Faith. I always had a thing for evil Willow 😅 (*cough* S3E16) Although, some great stuff happens in the later seasons. Like when Harmony turns vampire, tries to boss minions around, and her hilarious relationship with Spike. Willow getting powers is a pretty epic character arc, too. Dang, it's probably time to boot it back up! Never did get into the Angel show, so I probably missed a bit of crossover story.