I would say the following:

1) Flying lap
This is raw speed, and can be measured by how many pole positions the driver has, fatest laps in race, who was his/her team mate, and consistency over the years (eg: he could had the best car in one year, and had no pole in other years).
2) Start
The great driver must know how to start a race. How many overtakes is able to do in the first lap. How the driver can avoid contact and not get out of the race on the first lap.
3) Overtaking
This is what racing is all about, and what spectators are hoping to watch in every grand prix! The real driver must know how to overtake: from the inside, from the outside, low speed corners, right speed ones, and etc.
4) Rain
The best driver of all time (Senna) was the master on a wet race! So, the driver that wants to become the “second”, must have great skills in rain.
5) Race Control
Not just manage the tyres, but here is the complete skill set to manage kers, DRS, brakes and etc.
6) Setup
Every great driver knows how to work with the engineers and mechanics to get the best of their cars. That is one of the key differences we can see while comparing two team mates performances over the year.
7) Championships
There is no reason to be the best if that does no result in winning championships. So, the number of championships that a driver has on his/her pocket, is a key point.

Well with the actual Formula 1 (six champions), who gets the best score?