I've got to go with el Kun, he's helped Atlético to a CL-spot, but I think Messi is half a level above everyone at the moment.
Javi Martinez is a superb player and will soon be chased by big clubs but I still hope he'll stay with Athletic. Markel Susaeta had a great start and even though he faced problems throghout the season, he still managed to keep a spot on the team, a personal favourite.
It's safe to say that Diego Capel and Bojan Krkić got their real breakthroughs this season, the same goes for many other players, but I've got to go with Agüero.