Sarah, giving up the potion that would've turned her human to save Ethan from being a werewolf.
Then later in the season finale, Sarah giving up her final chance at humanity and forcing her to become immortal after Jesse bites Ethan, forcing either Ethan to become one, or her.
The season 1 finale, when Sarah, Rory and Erica are outside of Ethan/Benny's house, comforting Sarah and congratulating her on becoming immortal.
Ethan and Sarah going on their date in Pt 1 of the Season 2 Finale.
Benny's grandma telling Ethan and Sarah that regardless of whats going on, they care about each other and Sarah is the one Ethan wants to protect most.
It's quite the subtle one, but in the episode "Flushed," Erica is treating freshmen in a way that qualifies as hazing. When Sarah starts getting massive gas from over-feeding on blood and runs from the room however, Erica shuts up the freshman who were loudly complaining about the smell with a forceful "What are you looking at!? On the floor!" This could be interpreted as a simple finale to the general joke, but it's arguably Erica sticking up for her best friend, and the contrast makes it even more touching.