I'm going with Stephen Root, he has just done so many roles. From Milton in Office Space to Fuches in Barry the guy has a hell of a range.
Steve Buscemi is pretty good.
Also a fan of Neil Maskell. Check out Kill List or Utopia
Chris O'Dowd. In every role he sounds and looks the same BUT he's not the same character.
As opposed to someone like Clint Eastwood who is basically the same character regardless of the role.
I am fan of frances mcdormand from 'Fargo'.
Jim Lahey from "Trailer Park Boys". IRL he was just a super duper nice guy, but played the drunk trailer park antagonist to a key where he outshined the main characters of the show