Hard to choose, but I think I will have to go with Key/Joyce's Illness one-two-punch from season 5. I think one of the reasons 5 is my favorite season is because it's focused purely on the Summers women, Buffy as a hero and the Scoobies coming together as an adult family. And, despite the fact that I'm a big Bangel shipper, the lack of romance for Buffy this season is a big part of the appeal. Just Buffy and Dawn.
Following that, the Angelus and the Faith/Mayor arcs are very high up as well. Season 2-5 was definitely the show at its peak, despite the Initiative storyline, and even that wasn't so bad compared to...other things.