I must say for me, it is a difficult choice between Buemi and Kobayashi. Before today's race, i would have said Buemi but i think Kobayashi just by his enthusiasm and lack of fear shades it. That overtake on Button was amazing. His second race and he beats a veteran like Trulli. Good stuff! Grosjean has been a disappointment, although it is difficult really to judge knowing how Renault has been poor this year.
I must say , this is the most difficult year for a rookie. No testing and you are expected to get in the car and race. Kobayashi is really a revelation.