Who is the best driver in Formula 1? Lewis Hamilton? Sebastian Vettel? Valtteri Bottas? Kimi Raikkonen? Jenson Button? Daniel Ricciardo? Daniil Kvyat? Max Verstappen? Romain Grosjean? Nico Hulkenberg? Fernando Alonso?
It's impossible to know for sure, because of the technical discrepancies between the different constructors in F1. It is possible to extrapolate through various comparisons over time, to get a rough idea, but a true comparison is fraught with caveats.
It's fun to analyse, debate and guess, but certainty is always evasive. Take Alonso, for example. He is a double world champion who has toiled in uncompetitive cars for the past two seasons. Last year (with Ferrari) he reckoned to be his best in F1 in terms of his own performance - crushing world champion team-mate Raikkonen.