Kubica if he was racing, but as he isn't it has to be Rosberg or Heidfeld, currently we don't know the speed of the cars so it's difficult to speculate. However going solely on my opinion of the drivers, the best of the rest is Rosberg.
I wouldn't be so quick as to put Massa in the top six. I'd say that Rosberg, Schumacher or given the right circumstances Heidfeld are more than able to challenge for the sixth place (or maybe even 5th) on that list.
Judging by the new McLaren so far it could be hasty putting Hamilton and Button in there...
I feel I have to mention Rubens, Williams have a genuinely radical car, following Newey's Red Bull philosophy but taking it even further than Adrian has; it might all blow up in their faces (metaphorically or literally) but if it works they might spring a big, big surprise.
Who knows? Like Galahad, I dont think even the top 6 are sorted yet. But am going to say Barrichello, ive had a feeling early on Williams might just suprise people. but at the same time it would be impossible to rule out Kobiashyi, Sutil, Hiedfield, Rosberg even Schumacher maybe? Theres so many teams and drivers with potenital. And i don't think theres much of a natural order anymore with who will head the midfield like perhaps there has been in the past. Although it does seem so far that perhaps the 'best of the rest' class will be higher up the field this season.