I picked Simon because I think he has made a lot of progress:
I said Del Potro because he won those tournaments in a row...but I was tempted to say Simon.
he played 3rd round in ao against Rafa and lost 75 62 63, then he made a lot of progress, the simon of the start of the season and the simon of the middle/end of season are like night and day. I also like his attitude to never give up.
IIRC Del Potro is the 2nd player in atp history to win 3 tournaments in 3 weeks. (previous was Coria).
I think both of these players were surprising to me. I mean, maybe they were saying more things about Del Potro so he was expected to be on the rise. But you're right about Simon. Even now when I watch some of his matches I still don't see how the heck he wins...but he does. I think he practically came out of no where to be the #1 French player and that's no easy feat. So yes...he was a surprise. He passed Gasquet, Tsonga (expecially after the way Tsonga started out at the AO)... So he was definitely a surprise.
Del Potro needs to "grow up", maturity, he doesnt do well in the most important matches. he will improve.
then Nishikori who was ranked outside the top 200 at the end of last year, he is now around the top 60. Very big surprise but I have seen him and he has a long way to reach the top 20, he is very young.
Cilic, you know the tradition of the ao surprise finalist, well, Marin Cilic is a great candidate to be the surprise of the AO. I like him a lot, the future star of Croatia.
A very interesting year with young players to follow closely next year.