I saw a thread like this on Digital Spy so thought I'd put one here as well. I'm going for Stella. At first I thought she was scatterbrained, immature and well meaning but then she hid from Ben in the London Dungeon and I thought that was nasty and she was weird but I hated her from the moment she pressed that hot spoon to Ben's hand - evil. In May's case despite agreeing what she did to Dawn was wrong, I still have sympathy for her and she's still one of my favourite charecters.
So I vote Stella as best villain.
What do you think?