Jeffrey Combs.

I know that he doesn't play one main role but he has played so many roles in trek alone, and 2 of my favorite trek villains in weyoun (and his clones) and commander shran, that i consoder him a main actor at this point.

Also if you have watched a good deal of Sci-Fi in the last 30-40 years, you have seen him there too.

I am gonna leave an excerpt from his Mem. Alpha entry here, i recommend reading the whole thing, he has done a lot.

Combs has since earned himself quite a number of appearances on Star > Trek, guest starring in thirty-one episodes of Deep Space Nine, one episode of Star Trek: Voyager, and eleven episodes of  Star Trek: Enterprise. In "The Dogs of War", Combs played both Weyoun and Brunt, becoming one of only three  Star Trek actors ever to play two unrelated characters in the same episode.

He is one of only five actors to play seven or more different characters in > the Star Trek franchise, the others being Randy Oglesby,  J.G. Hertzler, Vaughn Armstrong, and Thomas Kopache.>