I wouldn't consider that as relevent as the fact that it really isn't fair to count Doohan's voices. A voice is much easier than playing a character (you can't play many different characters in the same episode like you can with voices).
Actually, the most reused actor - in pure terms of hours of Trek he's filmed - is unquestionably Michael Dorn. Granted, he spent almost the entire time as the same role, but the OP isn't 'Actor with Most Trek Roles', is it?
^ In "The Dogs of War" Jeffrey Combs played both Brunt and Weyoun, his two recurring DS9 roles. There is even a point where they cut from Brunt grovelling to Weyoun speechifying. It is one of the only times one actor has played two completely unrelated characters in Star Trek in the same episode. (The other time was "The Defector", where Patrick Stewart played a holographic Shakespearian actor that Data played off at the start).
ok So it was Jeff Combs.. Cool. I read somwhere that he said weyoun was his favorite charachter out of all of them he played.