Everyone has their own personal criteria for assessing one driver against another. What's more important, raw pace, race pace, racecraft, ability to handle pressure, courage, or risk management? Those are just a few categories that can be considered, but rarely do we see any one driver checking all the boxes.
We could go on forever pursuing the perpetual bench-racer's argument on whether one racer is better than another, but it's like calculating the last numeral in pi, it just goes on forever. We've all seen the debates (for example) where one person states that Alonso is better than Hamilton because ... yadda yadda yadda, and the retort of "yabut", and then a response listing different characteristics. And we've seen drivers who are absolutely unbeatable in a car to their liking, but struggle mightily when it isn't. That is just one example where one driver looks foolish in comparison to his teammate, despite a rational and logical explanation for the difference in performance.
Even when I look back to the history books it sometimes doesn't seem to make sense. Alain Prost definitely is one of the greats, yet he won more races that he obtained pole positions. Does that make him a slow qualifier, or is it relevant that many times he was up against Senna, who was willing to risk it all to qualify on the pole? Even when it comes to Fangio, arguably the greatest Formula One driver of all, in what is arguably the greatest performance of all time, the 1957 German Grand Prix at Nürburgring, started on pole with fuel tanks half full, yet was passed by two other cars at the start of the race. A slow starter?
So all this leads to my personal opinion that just like a car, it's a series of compromises, where some characteristics are traded off against other characteristics to wind up with a driver who finds success. Who knows, maybe Grosjean has it in him to be the greatest driver of all time, but right now his priorities are not optimal, maybe if he prioritizes risk management over aggression at the starts we would see improved results. And even that is a fluid and changing situation. People change, most of us learn from our mistakes and improve. Hamilton had a disastrous year in 2011, yet 2012 was one of his best ever, he was a changed man.
Personally, I gave up on this debate, all I look for is a driver who finds the correct balance of priorities to find success.
Racing should be decided on the track, not the court room.