There are way too many awesome moments for me to choose. I saw it on DVD a few years after it was off TV, and I loved the show immediately. I guess I could mention my favorite episodes (Tabula Rasa and Once More, With Feeling are tied for number one) but even then there are so many awesome episodes and moments, it seems unfair to just mention a few. Still, just off the top of my head (not in any real order, just as I think of them)
1. Willow going crazy, and Xander being the only one who could stop/save her. That whole scene where he has to talk her out of basically destroying the world was great, especially after Tara's (very surprising, to me atleast) death.
2. The whole last episode was epic, and had great action, dramatic, and sad moments (its a crime that Anya died but Faith, the worst character in either Buffy or Angel, lived)
There was so much more I can't even begin to list. It was an amazing show (as was Angel).