Pregnant Darla absolutely blew my mind. That was WTF in the most awesome way.
The only things that really stick out are Spike being the Doctor and Xander summoning Sweet. I can head-canon the first one pretty well even if it was badly foreshadowed and written. The second? Uh, no, Xander just wouldn't. The guy has his faults but messing with magic is a mistake he would not repeat.
Oh, also Buffy telling Giles that only Spike had her back. I mean, wtf? But that was season 7.
Cordelia killing Lilah!