21: The Killer In Me (Season 7, Episode 13)
An underrated episode but arguably one of Season Seven's strongest. I'm not a big Willow/Kennedy shipper but I did like them more in the series than the current eighth season comics book and this was a nice episode for them. Plus the whole Amy making Willow subconsciously change into Warren was pure nastiness on her part.
22: What's My Line Part 2 (Season 2, Episode 10)
Not a particularly pleasant episode for Angel. He spent most of it bound, gagged and being tortured by Spike and Drusilla when they weren't using him to restore the latter to full health. There's also some great scenes between Buffy and Kendra as the two of them try to iron out their differences with varying success.
23: Consequences (Season 3, Episode 15)
It's probably not that much of a coincidence that outside of Joss Whedon, Marti Noxon often wrote some huge character shifting episodes. This one finally saw Faith embrace her dark side when she decided to pledge her allegiance to the Mayor after Wesley's failed attempt to ship her back to England.
24: Who Are You? (Season 4, Episode 16)
What a brilliant episode. First off all, the reverse roles that Sarah Michelle Gellar and Eliza Dushku had to play were excellent, both were good in them but as a Willow/Tara shipper, this was the most blatant episode to show that these two were more than just friends. Also it would take Joss Whedon to use magic as a sex metaphor.
25: Welcome To The Hellmouth (Season 1, Episode 1)
The very first episode of the series (unless you want to count The Movie but that had a different Buffy folks) and one of the best too. Granted it doesn't look good now and you can certainly critique the attire of the characters but it did give us all of the Scoobies and introduced both the Master and Darla perfectly. And Luke had such potential.