Oleg Taktarov was the name I came to post after only reading the thread title.
Good choice. The only choice.
I've never seen Jardine really deliver a line, but he has always done a good job of adding an effective threatening ambience.
Rousey, Couture and Carano are actually the three worst actors I have ever seen from the ranks of MMA, in that order, slightly edging out Rampage.
What little I have seen from BJ Penn, Bonnar and Rich Franklin is bad, but they were also in absolute shit films with shit direction.
I actually enjoyed what little I saw from Teila Tuli, and while it wasn't a challenging role, Joe Son (yuck) actually did okay with what he was asked to do in Austin Powers. What a piece of shit, though.