The guy who played Brick Top in Snatch was only on screen for maybe 15 minutes total, but is one of the most memorable characters.
Robert Carlyle in Trainspotting is another good one. Whole movie changes as soon as he shows up.
Jon Hamm in Baby Driver.
Great choice. He even made that confused squint thing it seems like he always does seem like someone might die a second later.
Joe Pesci steals almost every scene.
Pesci might be the best part of The Irishman. The scene where he tells Frank that not only is Jimmy going to get killed, but Frank has to do it gets better on a rewatch when you notice how exasperated he is with the whole situation, but also realizes that decades down the line, he has to check Frank to see if he'll be a problem too.
And near the end, when he gets wheeled off and never comes back, as if he knows he's going to die, but somehow caught a W because he didn't snitch, and went when he was ready.