Ugh, this was so HARD! Then I had to think of which scenes are truly the most memorable for me. So...
1. "She Saved the World a Lot" Probably one of the most memorable season/series ending scenes EVER - the heroine dies saving the world. The music, the writing, the acting, the things we see and hear...it all is just epic. This is one of the first things I think of when BtVS comes to mind. It rolls what a Slayer is all about into one ending.
2. "Class Protector" This is another very memorable scene for me. Buffy finally, FINALLY get recognition and appreciation from the people she has been saving for years. She gets to be remembered. She gets her pretty dress, to be with her friends, and to dance at her prom with the man she loves. A rare happy Buffy moment!
3. "Au Revoir, Connor" There are few scenes in the two series that wrench my heart as much as this one. I don't know if it's because I have kids myself, but I don't know if I've ever felt as bad for a character as I did for Angel in this moment. I can't imagine what it would be like to watch your son that you love so insanely much be taken from you and then out of this world. I felt terrible for the rest of the day after watching this ep.
4. "Betrayed" This was a big "WHAT?!" moment for me, and a great display of SMG's superior acting skills. Through all this, you never would have considered Giles being the cause of Buffy's weakness. Definite plot twist, and change of character dynamic.
5. "My Respect" One of those calm moments that is just so poignant. Giles comes through and shows Buffy he is there for her, has her back, and will always respect her. I feel like this carries through most of the series (except S7 where ALL the characters had personality switches); he might question her decisions or disagree, but he truly listens and respects her thoughts and plans.
"We're Not Alone in This" is a very close 6th place.