I really like how James Doohan did so many voices for TOS. His Scottish accent for Scotty was so realistic, it's easy to believe that this was his true accent and that he didn't have any others.
Listen to Billy Connolly or Craig Ferguson if you want to know what a real Scottish accent sounds like. Whatever accent Jimmy Doohan was doing, it sure as hell wasn't Scottish.
But then, I've never heard a Russian speak English the way Walter Koenig did when he played Chekov. "Keptin, the alien wessel just wanished!" WTF?