s6e17 Normal Again messes with my head
It's just such a believable alternative that Buffy is actually a mentally ill young woman struggling with delusions of being a superhero that fights evil. Every time I've watched this episode I can't help but scratch my head like, "Huh...that actually makes a lot of sense..."
I don't believe that the whole show is actually Buffy's delusion but still, I think that's a testament to how well done the episode is. It gets me every time.