My favorite villains are enemies that

1) are hard to kill. No point in having a villain go down in one round.
2) have a variety of abilities. A warlock blasting the PC with the lowest hp until he goes down might be effective, but it's not fun for the players
3) are intelligent. I have to be able to justify pre-planning, good strategy, and clever combat.
4) have a human touch. Maybe he's a good father. Maybe he tells funny jokes. Maybe he's courteous to a fault. While he's undoubtedly evil, this trait makes the PCs wonder.

Lloyd Brown