Vettel, Alonso, Hamilton and Kubica. There's very little between them. Vettel gets my vote though simply because I like him more than the other three. 
I think Alonso and Massa will be closer this year, maybe a wins worth to Alonso ahead maybe more. Just remember what happened to Raikkonen, he beat Massa in year 1 got thrashed in year 2. Could it happen again? Could the tyres not suit Alonso? Last time we saw drivers having to conserve their tyres, Alonso was ontop by 21 points.
For the tyre preservation reason alone, I'm going to say life will be tough at McLaren, Button should be better in the races but not as strong in qualifying.
My Opinions
Best All Rounder: Alonso
Fastest: Vettel
Best overtaker: Hamilton/Kobayashi
Best attitude: Rosberg
Best under pressure: Alonso
Best leader (champ, not team): Alonso/Schumacher
I pretty much agree with that although maybe Alonso and Hamilton with Vettel for fastest; remember Monaco '07? They lapped everyone but Massa and he wasn't far from it...
what I dont understand is why Juan Pablo Montoya left F1 to go to Nascar mediocrity
McLaren gave him a dull car compared to it's predecessor and I guess he just lost his passion. I don't really know to be fair