This is dedicated to those actors who can make you love them regardless of whether their character is loveable or despicable.
To start:
Lee Marvin. I've never seen him play a role I didn't love, even when he was throwing boiling coffee in women's faces, leading The Dirty Dozen, or clearly drunk out of his mind.
Alan Rickman. Never failed to completely charm in any film I've seen, from Hans Gruber to Colonel Brandon.