Buffy was only good for the first three seasons
I used to think like this but it´s just not true. I mean sure, season 2 gave us Angelus and Spike and they were both great and as cool as vampires can be but honestly the romance was just super corny and cringe even if we wanted it to be this tragic and impossible love story. Buffy is just not at her best as a character when she´s playing the teenager in love.
Now S03, that was interesting because instead of pretending nothing happened Buffy had to deal with the consequences and contradictions of her actions. Running away from home, having to work on mending her bond with her mother and friends, having to deal with the duality of what it means to be the slayer (Faith gave this a completely different dimension compared to the prophecy or Kendra)
S03 had a great balance of action, character development and great villains, plus it had vampire Willow but i digress. It´s clearly above the first 2 seasons easily but it´s only possible because of those seasons set the field for it AND it´s only just beginning. The characters arcs only become better and better after the end of high school.
Now you could certainly argue the quality of the antagonists goes to the gutter and i guess that is kind of fair but the character development compensates for it in spades. I mean, Faith redemption arc is great, Weasley´s descent and return from darkness, the episode on which Buffy has to deal with the death of Joyce or even the episode on which she sacrifices herself for Dawn. That one hits like a break and far harder than Buffy´s death on season 1 even though we could agree that the Master is hundreds of times the antagonist Glory will ever be.
And yes, i am counting stuff from both shows simply because some arcs are completed on Angel so they are kind of part of the same thing. Which is, i believe, the real strength of the whole thing.