I'm pretty sure it's Vaughn Armstrong - he's played something like 12 different characters. I thought he had said it was over 20 when I met him a few years ago, but IMDb only has 12 or 13, depending on whether his two appearances as a Kreetassan captain on Enterprise were intended to be the same character or not - I don't recall if anything was said in either episode to indicate either way.
(And yes, I'm counting Captain Forrest in "In A Mirror, Darkly" as a different character from Commodore/Admiral Forrest in the rest of Enterprise.
It's probably well over 20 if you count all of the various video game voices he's done.
OTOH, Jeffrey Combs scores extra points for playing two different characters in the same episode.