Okay, I'll start first. For me it's Peter Hale. From the very beginning he knew exactly how to play people, and the way he manipulated Lydia was both terrifying and brilliant. He wasn't just some random bad guy, he had plans within plans, and half the time you didn't even realize until it was too late.
And the best part is how his role kept changing through the seasons. He could be a villain, a survivor, an ally, even a backstabber, and somehow he pulled it all off. One moment you hate him, the next you find yourself enjoying his scenes. I especially loved when he was with Stiles. Their interactions were always hilarious, full of sarcasm and those quick lines back and forth.
So that's my pick, but who's yours? Which male character had the best development for you?