I'd say Mickie, or the pretense that she should (or could be) diva of the year. She is clearly the most over. However, she isn't, because that title undoubtedly goes to Beth Phoenix. It's not even really debatable.
Beth Phoenix, has, and almost certainly will hold the title up until Wrestlemania. Certainly a good thing as well, she has given the divas title a built up amount of credibility. If I was booking for the WWE (God knows WWE doesn't want to become interesting), I would have her keep strong until mania, then lose it to Kelly Kelly cleanly. That'd be a great fued.
Anyway, Beth no doubt. She is the strongest women title holder the WWE has seen in ages.