I've out some thought into it and I guess I'll give my top-10 here.
HM: The Body.
It doesn't make my list, not because it isn't a terrific episode of TV, but because it's essentially the non-Buffy episode of Buffy. It intentionally doesn't deal in what Buffy is and that makes it stand out, but it also doesn't encapsulate what I love about the series. On merit it is easily one of the best episodes, which is why I'm mentioning it without including it on my list
10: Prophecy Girl
Season 1 is generally very rough, but it's finale is maybe the strongest performance of Sarah Michelle Gellar's career. Buffy breaking down when she learns it's her destiny to die, as we're reminded this is a 16 year old girl with all of this heaped onto her, breaks my heart every time. This is the episode Buffy became Buffy
9: Conversations with Dead People.
Season 7's best episode IMO. A strong bottle-episode featuring three conversations between dead characters and living characters, although two of the conversations are deceptive. It serves as a creepy re-introduction to the last big bad and has a terrific performance from guest star Azure Sky
8: Passion
David Boreanaz's narration here is a bit on the corny side, but the idea of this former-hero/now-villain taking an almost sexual pleasure from the grief caused by his murders really lands and raises the stakes higher than they ever were before. This is the episode where Buffy let it be known that it would kill major characters, at a time where that wasn't the norm for network TV. The Angelus arc in Season 2 of Buffy is probably the most fully realized in all of the Buffyverse and this is one of the biggest episodes.
7: The Gift
Though I'm not the type of guy who believes that bittersweet endings are always better than happy ones, and I certainly don't believe Buffy would be better if she died at the end, this episode is all-around very strong and could have served as a great series finale. Buffy/Glory's final confrontation was excellent, it had many great character beats - including Giles murdering Ben and Spike and Buffy's excellent stairway conversation, but it's the emotional final shot that gets me every time. She saved the world. A lot.
6: Graduation Day
What is it about Torrence High School that makes the graduation episodes shot there so terrific? This one perfectly captures the feeling of renewal and the energy of graduating, and it's an excellent ending for not only one of the franchises best villains, but the high school era of Buffy.
5: Hush
Easily one of the most well-executed episodes in Buffy's history. Creepy, funny, great effects and good mythology. If you haven't seen this episode: see this episode.
4: Restless.
The most brilliantly stylish and unique episode of Buffy. Joss loves him some metaphors and this episode combined brilliant weirdness, epic foreshadowing and referential heaven.
3: Becoming Pt.2
This episode has it all: The first ever Buffy-Spike team-up. The big confrontation between Buffy and Angel. A strong beginning involving being framed for murder, Buffy being outed as the slayer and an ending that involved the series' most powerful non-OMWF musical moment.
2: Innocence.
The great heel-turn in TV history. The most exceptional genre busting, rocket launching climax of TV. If Buffy is a feminist high-point, this is the episode that cemented that.
1: Once More With Feeling
It's one of the best episodes of any TV show ever. Fantastically shot, beautifully written with near-Broadway quality music and more character development than any other two episodes of Buffy ever. This is the musical episode that all other musical episodes answer to.