Wilhelmina Slater is one of my all time favourite villains. She is classy and well-dressed, and a stone cold bitch. I love the way she pretty much owns the show "Ugly Betty".
If you didn't know, and I'm spoiling it for you, TOO BAD! WILHELMINA GOT SHOT! She can't leave, she just can't! The show would be nothing without her. Mark and Amanda's little stabs at people are humorous but nothing in comparison to what Wilhelmina does.
And even though Betty's wardrobe has improved, one of the main reasons I watch the show is for Wilhelmina's fabulous wardrobe (plus a little bit of the drama ;)).