16: Passion (Season 2, Episode 17)
If you're going to turn Angel into thr Big Bad for the second season, then at some point he was going to have to do something truly awful (and trying to end the world on this show doesn't count). Having him kill Jenny Calender was far the most shocking he's ever done as Angelus during Season 2.
17: Fool For Love (Season 5, Episode 7)Crossed over with the Angel episode Darla, this was without a doubt the best crossover story the two series ever did and this one gave us a nice look into Spike's penchant for slayer slaying while he became more and more obsessed with Buffy.
18: Smashed (Season 6, Episode 9)
19: Surprise (Season 2, Episode 13)
There are plenty of things you might want for your birthday but dreams of your boyfriend dying and Spike and Drusilla wrecking havoc ain't two of them. It also doesn't help that by the end of this episode, Angel's reverted to his original form.
20: Angel (Season 1, Episode 7)