I'm going with Daniel Bryan as well. As has been pointed out, Bryan has gone from being a primarily generic babyface mid-card wrestler to one of the most over wrestlers on the WWE roster over the course of the past 12 months. He's gone from someone that many were expecting to be future endeavored at just about anytime to someone that's shown he has what it takes to be a long term star in WWE. Bryan winning the WHC and having a significant run with it was as pleasant surprise as well. The only complaint I have about Bryan's run is how it ended, but I can't argue with the aftermath in that Bryan is a much bigger star after he held the title than he was before he held it.
Sin Cara has improved a lot in the ring, he no longer botches all over the place, but he's still the same generic luchadore that he always was.
Big Show has improved a lot, I do have to say that. He's been consistently entertaining throughout this year and his matches & feuds with Daniel Bryan & Sheamus have been great.
Austin Aries, all in all, hasn't really shown me that he's improved. He's still great but the only difference really is that he had a brief run in TNA as the WHC. That'd be great if he hadn't been completely overshadowed by Aces & Eights and generally put on the backburner.
To me, Samoa Joe is pretty much the same as he's always been. He gives a few heated promos now and again but, all in all, more of the same.
Kane has improved a lot but, frankly, I think it's more due to his invovlement & chemistry with Daniel Bryan than anything else.