I'd also add "The Body" to that list. A few months before I got into Buffy, I was with my Stepfather when he died, and my main reaction watching that Episode was "Yeah, it was just like that." I've never run across any other work that captured and presented that experience so faithfully and completely. An absolute masterpiece.

Not just in and of itself, either. I've got the outline of a post in my head that I haven't gotten around to writing up about how the last scene, with the vampire, lays out the themes for the whole remainder of the show, particularly Season 6.