Season 1, Episode 1: "Pilot"
We can't talk about the top PLL episodes without including the very first one. What happens in that introduction sets up the next 159 episodes, and there are key pieces of information that play a role through the whole series. Right away, we find out that Spencer was in the arms of her sister's fiancé, Hanna has a shoplifting habit, Aria made out with her high school teacher Ezra Fitz (Ian Harding), and Emily is drawn to another girl and figuring out her sexuality. Sounds like the kind of things you would want kept on the DL, right? Well, A, in all of his or her creepy magic, knows all of this and uses it against them. And when you have a secret like that, you'll do just about anything to keep it that way.
The first text message sent after Alison's funeral services sparks what we can confidentially say is a whirlwind of a series: "I'm still here, bitches. And I know everything. –A"