Leon setting off an explosion that kills Stansfield after saying "This... is from Matilda." Also borders on Crowning Moment of Heartwarming, in a way. Bonus points for doing it after getting fatally shot in the back.
The entire opening kill.
"Somebody's coming up. Somebody serious."
Just to elaborate a little, we don't even see Leon fully until all the target's bodyguards are dead, and he just materializes out of a shadow behind the guy. He takes out the guards, blocks the exits and herds the target exactly where he wants, all from the shadows in a way that borders on the supernatural.
As the police invade Leon's apartment, it appears to be empty... until his hand closes the door. From the ceiling.