Hate to go back to the Buffy well, but I walked through the room as my roomates (who are watching the series for the first time) were watching this masterful episode:
73. Buffy the Vampire Slayer S2E17: "Passion"
If one was to pinpoint when Buffy moved from being a fun genre show to one of the most important dramatic shows of all time, it would be with the loss of Angel's soul and the emergence of the evil emergence. This arc produced four episodes that could certainly make this list ("Surprise", "Innocence", and "Becoming Parts 1 & 2"), but my personal favorite is "Passion". Joss Wheon has rightfully earned a reputation as a character killer; this episode was the beginnings of that hallmark. The death of Jenny Calendar, though, is so perfectly executed that I can see why he settled on the hallmark. The actual death is staged exquisitely; I have always loved the shot of Angel and Jenny framed by the dark window as he gleefully twists her neck. However, it is important because of its place in the seasonal arc. My roomate and I were talking before he got to this point in the episode, and he felt that, though David Boreanaz was giving a great portrayal and Angel was surely "evil", he felt that he hadn't come off as much more than a dickish ex-boyfriend. His murder of Jenny gave a palpable threat to Angelus that no other Buffy villian, in my opinion, ever matched.
I also have to give huge props to the effective application of voice-over monolauge in the episode. In general, I tend to agree with the sentiment that voice-over is a lazy crutch writers use to evince internal though without having to go through the trouble of externalizing it visually. No such problem exists in "Passion"; the voice-over just enhances the excellent visual composition of the episode. Angelus watching through the window as Buffy and Willow break down in response to the news that Jenny has died is a powerful scene. Here is the man who has been a part of Buffy's life since "Welcome to the Hellmouth", and now he watches her through the window as he tortures her. Powerful stuff, and Angelus's ruminitions on the power of passion (given gravitas by Boreanaz's excellent line reading) only add to the scene.
Key Moment: Giles returning home to find Jenny's corpse. As great as Boreanaz is in this episode - and he really is, it is no coincedince that it was this episode that earned him his own spin-off - Anthony Stewart Head was on another level. His smile as he picks up the bottle of champagne quickly turns to horror as he sees the lifeless body. Once can see almost moment by moment the excietement turning to dread turning to horror. The match cut of Giles horrified face staring at the corpse to him staring off into space as the police make their way through his apartment is the perfect capper. As horrible as Angelus's deed is, it is the "wrapping of the gift" (as Xander puts it) that really puts into sharp relief just how dangerous Angel has truly become.