The very first episode I watched all the way through was OMWF and I thought, "Oh dang, if the music for a one-shot musical episode can be this good, the entire show has got to be amazing." And I was right! =P
I remember in The Witch:
Giles: You are the Slayer! Lives depend upon you! I make allowances for your youth, but I expect a certain amount of responsibility, and instead of which you enslave yourself to this, this...cult."
Buffy: You don't like the color?
It was so good to almost make fun of itself! Like, Buffy is this young girl with this burden of the slayer so they won't have her wanting to be normal or want something like cheerleading, but she did! And it felt so real. This was a pivotal moment for me.
Also, all of season 2. Season 1 was cool and it had a "Hey, it's the 90's and Buffy is a slayer and girl power, yay!" And by the end of season 2 with all the tragedy and death and heartache, I thought it was such a testimony how the show was willing to go there and be deep and not just stay as some surface high school drama of a girl who kicked butt.