I'm in the middle of a big Buffy rewatch atm, just finished Season 2, and whilst I have almost certainly raved on about what a great episode "Passion" is, I have to say it again- I'm not sure that these are favourite moments in the sense of being great fun to watch, they're very depressing, but they're so well done that they stick in the mind:
1) Giles finding Jenny's body: Just... the way the scene is set up. We know that Jenny is dead, we therefore know that she cannot have set this up, so seeing Giles walk in, all smiling, seeing the soft lighting and romantic set up, we all know that it isn't what it seems, in fact when Giles walks in to that set up, the audience realises just how cruel Angel has been. And the way the music builds up and crescendos as he walks up the stairs, his happiness probably rising with every step, and a huge grin growing on his face, and the audience is getting more tense because they know what he's going to find, and then as the music reaches that crescendo, he reaches the top, he sees the body, his face falls, the glasses drop to the floor.
Just... it's horrible, and it's cruel and it's heartwrenching, but Anthony Head's acting, and the way the music is timed, it's just done so damn well. Most memorable moment of the series, for me.
2) The phone call to Buffy and Willow, with Angel watching through the window- again, very well done, I think- you only hear one side of the conversation, and that is muffled, it's Angel watching in, surveying the damage that he caused. Again, very well done.
So, yeah... at some point I will try and think of some happier, cheerier favourite parts XD