I will always remember the trilogy of "Apocalypse Nowish," "Habeas Corpses" and "Long Day's Journey," because the Angel Investigations gang got beat down and had their collective assi well and truly handed to them for three episodes in a row
. That cemented that they were in deep doo-doo for me - for them (the team, not the show) to have such a consistent losing streak was a remarkable thing, I thought.
Plus of course the awesome fight scene in "Apocalypse Nowish," which for me remains the best fight scene in the entire Buffy-verse. I sometimes pop the disc in just to watch that.