This is quite the toss-up. In terms of "favourite", I suppose you could say it'd be Jadis, especially in MN. She's... I don't know, evilly regal? Regally evil? Her complete disregard for human life is chilling, and her monologue in Charn, when she exposes its end, is... gah. That freaked me right out when I was younger. Now it just freaks me out somewhat.
Jadis's magic was never explored as much as I'd have liked, but I do find it interesting that she was so willing to use the Deplorable Word almost as soon as she got to Earth. She intrigues me.
In terms of which one I find most interesting, I think that'd be The Lady of the Green Kirdle. Her method for taking over Narnia was on a smaller, more intimate level than Jadis's, the Telmarines', or Rabadash's, and the break from the norm was appreciated. And of course there's the ever popular question, Where did she come from?
In terms of the vilest, though, that'd be Shift. I can't stand that monkey, at all. He made me so, so angry the first time I read it, and I still skip his parts and just read Puzzle's whenever I read LB over again. Tash is frightening, I suppose, but he's not the top villain (mainly because he doesn't have enough material to work with).