This one is way harder to do, number 1 is obvious but after that it becomes really hard to round out a complete list.
- Once More With Feeling (season 6 episode 7): This is the episode that really shows why BtVS is a great series. The songs sung by the Buffy and the gang all have insight into their development and really move the plot forward for the season and are very well done.
- Hush (season 4 episode 10): A lot of people say witty dialogue is the best part of any Whedon show, and Hush after the 13min mark til almost the very end of the episode, there is no dialogue. One of the best BtVS episodes.
- Grave (season 6 episode 22): The best season finale of any season in my opinion. Yellow crayons and Xander+Willow. So good.
- The Body (season 5 episode 16): Really great episode and really stark in comparison to other Buffy episodes and yet still is very BtVS.
- I Only Have Eyes For You (season 2 episode 19): A great Buffy and Angel/Angelus episode.
- The Zeppo (season 3 episode 13): I really like Xander episodes and this one is from my favorite overall season.
- A Fool For Love: (Season 5 episode 7): A lot of back-story on Spike and how it applies to Buffy, loved the ending too.
- Restless (season 5 episode 22): Definitely one of my favorites, it has so much character development and some great foreshadowing. and the Cheese dude.
- Chosen (season 7 episode 22): A great episode to finish off a great series.
- The Gift (season 6 episode 22): Second only to Grave for a season finale. I was completely floored watching it in the first run, as were my housemates that watched the show.
All personal opinion here, these are all episodes that I could watch any time any where and are great showcase episodes for the series in general.