What was the best performance by somebody not primarily known for their acting? #MovieDiscussion

For those not in the know, Ermey was only ever meant to be a consultant to teach Tim Colceri, the actor playing the drill instructor, how to do so realistically. It was only after Ermey did a 30 minute demonstration in which he berated and insulted the "troops" of the film while stagehands beaned him with tennis balls and oranges that Kubrick realized that casting Ermey would essentially just be removing the middleman.

You best unfuck yourself or I will unscrew your head and shit down your neck!

-- Gunnery Sergeant Hartman

I feel like Gunny is the real answer to this. He was funny as hell in Saving Silverman and the Texas Chainsaw Massacre movies.