A book doesn't necessarily need a villain to be a great book, but if you're going to have a villain, they must be compelling and AWESOMELY EVIL! Queen Levana is an amazing villain because she believes she's in the right yet she always makes the most evil decisions. I'd rather live ti hate the bad guy than be lukewarm about them.
Who's your favorite villain? I also love to hate the Commandant from An Ember in the Ashes, Voldemort (DUH), and Leann Frobisher from Illuminae.