I think to be
driver of 2015, a driver has to beat his team-mate. Even though unreliability is a factor, a drivers team-mate is still the most accurate measure. So that leaves half the field to choose from:
HAM-VET-BOT-KVY-PER-GRO-VES-NAS-BUT-STE
Personally i'll remove Stevens from that list, because I don't rate either of the Manor drivers very highly. I also remove the Mclaren drivers from my list of candidates, because honestly they haven't got a machine reliable enough to prove themselfes, and even when they both finished we can't be sure the cars were equal with the team running test-items or downtuned engines.
Next I'll remove Kvyat from the list: I think he did OK, but was very seldom better than Ricciardo. I think he got lucky to beat him. Besides, until Monaco he was pretty bad. Even after Monaco, he had some terrible weekends (Suzuka!).
Also i'll remove Nasr from my list: Didn't impress me that much this season other than in Australia.
That leaves:
HAM-VET-BOT-PER-GRO-VES
Honestly even though Bottas beat Massa over the season and had some solid drives, to me he just isn't that impressive. He seems very conservative, passive, defensive. I think he must do better and beat his teammate by a bigger margin if he wants a Ferrari/Mercedes seat. He is not a rookie any more.
HAM-VET-PER-GRO-VES
At this stage it's getting much harder to pick. Being really picky i'd say Perez was somewhat anonymous in the first half of the season. Grosjean has being very consistent, beaten his teammate, but only caught the spot light once (at SPA). Verstappen has been amazingly impressive, but made mistakes. Highly entertaining, but maybe not as efficient and losing some points here and there. So:
HAM-VET
Both have been super consistent. One could argue VET has beaten his team mate by a bigger margin, but then he probably had a lesser driver to beat. HAM made mistakes in Hungary. Vettel in Bahrain and Mexico. Vettel got to endure slightly more car-issues (Qualifying Canada, penalty for 5th engine in the US). Both got screwed by their teams to some extent (Hamilton in Monaco, although it was partly his fault, and Vettel in Abu Dhabi, although he should have done a better lap on the softs). Vettel also had to get used to a new team.
Conclusion. I don't know
