It seems to me that the car is the greatest determinant of the championships, and that it is therefore impossible to separate the performance of a driver from that of his car. Therefore, I would think that the most logical way to gauge relative driver performances would be by comparing the drivers within each team, and the driver that out scored his teammate by the highest percentage did the best job. Using that criteria, the rankings would be along the lines of:
1.Grosjean
2. Alonso
3. Sainz
4. Bottas
5. Hulk
6. Ricciardo
7. Vettel
I doubt that anyone else will agree with this assessment, but it does have the virtue of taking emotional attachment out of the reckoning.