Episode 1x02 -- "Night of the Comet"
"I met a girl. We talked. It was epic. Then the sun came up and reality set in. All this is reality. Right here." ~ Stefan
This is the episode where Stefan and Elena had their first kiss outside the Salvatore Boarding House and started their relationship. While it's easy to say they moved along rather quickly, this is an adorable scene. The music accompanying it, which is Sara Bareilles' "Gravity", was phenomenal and really set the tone for the couple.