As for Luthors, I'll cast my vote for John Shea as well. He was definitely one that was able to be smoothly persuasive and coldly menacing at the same time.
Shea was perfect for the romantic-comedy aspect of Lois and Clark. He had a certain charm that made you see him as a real rival for Dean Cain's Clark/Superman.
I just love the scene in the pilot where Luthor stares down the cobra and there's a tear running down his cheek. Shea just sells a lot about Luthor in that scene.