Going back to original question, if it's who's performed the best this season then that's definitely Kubica. When you take his car, consistency and results into consideration he's performed better on average than any other headline driver this year and also committed far fewer mistakes.
The accolade can not to Hamilton because despite some stupendously epic wins he's also dropped the ball on several occasions with some epic errors. Bahrain he crashes into Alonso, Montreal he loses the plot in the pitlane, Fuji he drove like a complete moron.
Felipe Massa has gone someway to proving some of his critics wrong this year, he's definitely developed a lot. However his spin at the first corner at Melbourne, his spin out of a safe second place at Malaysia, his five spins at Silverstone and his naive counter move on Hamilton at Fuji are more than enough to take the "best driver" tag away from him.
I'd still rate Alonso as the most complete driver in F1 overall, but he massively overdrove for the first half of the year, but he's now got that under control again.
Kimi's been erratic all year, he's a quality driver on a par with Alonso talent wise but he lacks the aptitude to develop the car. But that's just him, it's his style.
Other drivers worthy of a mention this year are Sebastian Vettel. Since Toro Rosso got that car sorted he's been terrific, but he also got himself into a lot of first lap trouble early in the year.
I don't think there are any others this year who could qualify as best of the year.
I also don't understand how so many people rate Sutil highly, in my view after Piquet he's currently the biggest waste of space in F1. I'm sorry but he just hasn't done anything... setting a fastest lap in a damp practice session does not set you up for future super-stardom and he has been out performed by Fisichella, a driver past his peak.
Lastly, I'm someone who rates Nico Rosberg highly, I believe he has good potential but I'd also have to agree that he's been very disappointing this year. He's still very quick but he's made a horrific number of mistakes this year. The Williams car in general has been a disappointment, mainly because it's winter promise got expectations up too much. but Rosberg has usually squandered his opportunities when the car has been competitive, like losing wings at Monaco, Montreal and Silverstone just three examples where he should have taken good results and a healthy amount of points. His drive at Singapore was terrific, but 1 out of 4 isn't good enough.