For me, the best quote is from season seven, "Dirty Girls", it's made by Xander:
"She's laid down her life-literally-to protect the people around her.
This girl has died, two times, and she's still standing. You're scared
that's smart. You got questions, you should. But you doubt her
motive, you think Buffy is all about the kill...then you take the little
bus to battle. I've seen her heart-this time not literally-and I'll tell
you right now she cares more about your lives then you will ever
know. You gotta trust her. She's earned it."
When I hear that quote and see the reaction on Buffy and even Faith's face, it makes me cry everytime. It really shows how sweet Xander is and how much he loves his friend-Buffy. It was just so touching. Finally, all of Buffy's work paid off and someone had realized all the good she had done. another good quote was on her tombstone: "She saved the world-a lot". I love that one, really speaks to who she was and continued to be.