Ahhh, good ol’ fashioned villainy.
I’d like to think the perfect villain is intelligent, whimsical, and has a (relatively) just cause. Y’know - Liberate the world form some ‘oppressor’ or whatnot, but doesn’t hesitate to shoot down some innocents for the benefit of the many (Kane, from the Tiberium Wars, I’m looking at you); a crime lord working in the interest of her family, and doesn’t care about who else it hurts; the monarch who is willing to sacrifice all for something ridiculous.
I feel that a good villain characterises fortitude, motivation and decisiveness. I always imagine a good villain doesn’t waste time by hesitating when meeting the hero. She just takes a shot and moves on with her life.
My favourite villains usually inspire feelings of empowerment and general badassery in me. Super bonus points if I’d rather side with their cause than the hero’s.
I think a style that suits their personality and modus operandi is in order - just like anybody else. If my villain is the type to have delusions of grandeur, I’d have him styled after an elaborate upper class. If my villain utilises mass propaganda and religious/political fear, I’d include some symbolism in her appearance and embodiment. The heartless, killing machine I’d style with only the necessities for his work, and act only to achieve goals - no unnecessary mannerisms or baggy pants.
Overall, I think a good villain should be alive in a dynamic world, pursuing their own agenda even if the hero fails to show up. Logically, they’d guard their secrets and only involve people they trust, so the hero shouldn’t even catch wind of them until their plans are in action.
I agree with @Spire:
All that said, Anti-heroes, in my convoluted opinion, only mar the bad name of good villains, and should go sit in the corner and think about what they’ve done.