Samson walking into the bar in Babylon in "Pick a Number" and proceeding to shoot Stangler right between the eyes, despite Carny Justice already having been dispensed.
In the 1st season finale: Ben, after realizing he'll have to take a life to save another's using his powers, goes through with it and slits his own throat as the exchange.
In "Insomnia", Samson manages to get one up on Lodz by giving him Scudder's watch and watching him convulse. Considering what Lodz has done up to that point, it's definitely worth it.
Ben meeting a father trying to sell his retarded daughter as a sex slave with the pitch "She's tight!" The rage in Ben's eyes and the sentence "Never again." is enough to reduce the man to tears.
Ruthie is pretty damn awesome, but tops herself in the finale with three words: "What a prick!"
Justin gets a bunch, but this troper has to give it to him for the entire Trinity dream sequence in "Los Moscos". Ben witnesses and withstands a nuclear blast, but then there's Justin crouching in the dust with his black eyes, quoting the gospel of Mark: Ye offspring of serpents, who warned you to flee from the wrath to come?.
Iris Crowe's confession to setting the fire that burned down her brother's ministry: I did it for you. Absolutely chilling, and her Crowning Moment of Creepy as well. The scene between her and Norman in "New Canaan, CA" where she says she's "going to hell" comes in a really close second, though.
Ben and Jonesy in "New Canaan, CA", talking about Sofie:
Jonesy: What if she's dead?
Ben: If she's dead, then God help 'em. God help every single person in that valley.
Another one from "New Canaan, CA" - Justin bringing the Ferris Wheel to a dead stop with two words: "BE STILL!". It's also the payoff to Justin's use of that power on a much smaller scale (in "Lonnigan, TX", with the patients in the asylum).