This year its really hard to judge who is best or not but Kubica imo hasn't got a team mate fast enough to be compared as the fastest however he is one of the top but last year he did get out scored by heidfeld, who i would rate as an average driver, Alonso was in the exact same situation in 08 .
My theory on Hamilton is he seems to be getting worse every year, i know this may sound completely stupid but think about it, in hes first year he was very conservative and barely made many mistakes and was very consistent, in 08 he made plenty of mistakes however so did hes competition so he could still win the championship, 09 was kinda unfair but the mistakes where still there, this year button is beating him on points because of un-consistency, Hamilton imo is probably the fastest on the grid but hes not consistent enough to be the best.
Alonso has made quite a few mistakes this year but is pretty lucky in getting points out of it, which alot of people kind of forgot, but Alonso hasn't been as consistent when fighting for the title since 2006.
Webber i think has been a Good driver hes entire career but once he got to the stage of being in a good car it was probably a bit too late however he would still be able to contend for a title.
Vettel is fast but he lacks consistency like Hamilton and seems to lack Race pace compared to hes Qualifying ability.
Button is a good driver but he was very lucky to get the title in 09 Imo, the car was built around hes style and was much faster then the competition he only really needed to beat hes pensioner team mate barrichello to win, even though in 2010 hes scoring good he seems to be just capitalizing on others mistakes or making good luck.
Judging by them all there pretty Equal. I would probably say Alonso overall is still the best but its close.
However Kubica is still abit of a unknown.