For me, there was two clear choices for this category, Cena returning at the Royal Rumble, and CM Punk winning the WHC on Raw.
As far as which surprise I enjoyed more, it was Punk winning the title. I'm not the hugest CM Punk fan by any means, but I do think he's a great wrestler and was pleasantly surprised and even a bit excited that he won the title. I wasn't completely shocked that it happened once Batista came down and beat Edge up as I kind of figured that this was leading to Edge losing the title to Punk, but it was still surprising to see CM Punk actually go through with winning the title at the same time. I just really liked the whole direction Raw went after that for awhile, with the whole no GM and complete chaos theme. It showed once again for the first time in a long time that "anything can happen in the WWE," and I personally enjoyed Raw the most during the summer with Punk as champion. Just because it was something new and different. Unfortunately the only guy Punk got to feud with was JBL, which was a horrible choice. Not only because the matches weren't very good, but also because it made people lose interest in Punk and his championship reign. I think if Punk had gotten to feud with someone of more quality like Jericho (who was unfortunately tied up in what was for my money the best feud of the year with HBK) or Orton (who was unfortunately injured), Punk's reign would've been more successful and perhaps more long term. Even a feud with Batista or Kane would've been more interesting and probably resulted in better matches than JBL.
But my vote didn't go for which was my favorite surprise, but what made me drop my jaw the most when it happened, aka: the biggest surprise. And that honor has to go to Cena returning at the Royal Rumble hands down. I was under the impression he wouldn't be back until after Wrestlemania, and here he is coming back months ahead of schedule with no build up or news on the net about it or anything. Just completely out of nowhere, which is practically impossible to do nowadays in pro wrestling. It was the biggest surprise that WWE has managed to pull off in ages. It wasn't my favorite surprise, but it was undoubtedly the biggest, which is why it got my vote.