In the pilot movie, when Ethan and Benny follow Sarah into a dark alley and discover her feeding on a rat. First, they make two incredibly girly screams, then Ethan runs as Benny continues to scream two more times until Ethan comes back to yank him away.
Sarah's defense against allegations of drinking from dug up corpses:
Sarah: First of all, gross, and second of all, eww!
Anastasia: Both are valid points.
Ethan and Benny are talking about a Halloween party, and Ethan considers inviting Rory, to which Benny responds by reminding Ethan of what happened the last time they invited Rory to a party. Cue flashback of Rory wearing a makeshift one-shouldered loincloth and a broom, leaping onto a couch and chanting "I'm a caveman!"
Benny: And that wasn't even a costume party.
Sarah's imitation of Erica choosing a victim to eat:
Sarah: He's too stringy, he's too old, I hate her outfit.
From the Pilot Movie, the scene where the vampires mock Ethan for needing a babysitter. Just the whole idea that a group of vampires would take time out of their initiation ceremony to make fun of their prey is absolutely hilarious.
"You potioned martial arts experts?!"
This exchange from the second season finale
Benny: You bit my best friend!
Sarah: And you made me suck out the venom when you knew what it would do!
Jesse: And I'd love to be sorry about that, but you all know I've done much, much worse.
This moment from the movie. It's Sara's tone on the last line that really sells it:
Sarah: Whatever you guys saw, it's not what it looks like. Ethan: You're a freaking vampire who kills animals and drinks their blood! Sarah: Okay, so it is kinda what it looks like.