What you're really asking for is a favorite among favorites. The series was amazing, but I'd still pick "The Prom," from near the end of season three.
Just a personal favorite for me: Angel breaks up with Buffy, leaving her devastated. But out of that. Buffy gets her big moment at the prom, as Jonathan calls her name: unexpectedly, her classmates recognize what she's done for them and award her the title of "Class Protector." After which Angel appears, decked out in his formal wear--just for a dance, but the gesture completes Buffy's perfect night.
This one still gets me all misty-eyed.