Lost: Two for the Road

Ana Lucia: "You tell anyone about this and I'll kill you."
Sawyer: "Guess that takes cuddling off the table."

A whole freaking season, practically nothing happens. And then, boom. Hot sex and multiple character deaths, all in one episode.

Lost: Collision

Sayid: "What good would it be to kill you if we're both already dead?"

Ana Lucia was a cop, and she's had a rough past. It fit her personality like a glove.