MVP is definitely the most improved imo, but he wasn't great for me anyway the whole year, I am coming around on him.
If you look at the whole year though I think you have to look at Batista. This was a big year for the injury bug and tista was able to stay healthy the whole year and put on a lot of good matches.
I can't vote for Edge or Taker because both were injured for the majority of the year. Hell Edge started the year on Raw and then got injured. Although both were great, they weren't out there enough to make me vote for them.