I hate having to choose favorite villains, because I always love the villains anyway. But its probably a tie between the Vashta Nerada (S4) and Rosanna Calvierri (S5). The Vashta Nerada are creepy as shit, but in the episode they were in they also had reason for doing what they did. Their forests were destroyed to make books and they had no other place to live. Similarly, Rosanna Calvierri and her kind had their homeland eaten by a crack in the universe and therefore had to find somewhere else to go very quickly. I may not agree with devouring/killing innocent human beings, but at least these guys weren’t the one-dimensional “I am evil because mwuahahaha fuck you” type of villains. They were interesting.