WDC are only one factor.
Its a very tough question to answer really, you need to take into account sooo many things.
Length of time in the sport - The ability to keep getting a drive, and maintaining performance over a number of rule changes, tyres types, engine types, driver aids, etc
Race wins - Clearly you need to be winning races to be a true great!
WDC wins - A good indicator of maintaining performance over a year (although there are caveats!)
WDC position when not winning - Always being in contention, even if not winning can show a truly great driver. Especially when driving a poor, or unreliable car
Performance against team-mates - The only almost solid and stable benchmark over a year. Consistantly beating a "decent" driver over the season is a great indication
Pole Positions - Out and out pace, speed has to be considered here.
Qually position when not on pole - Days when a driver did not quite hook it up, where where they?
Points finishes - Great indication of race-craft and competance
Race finishes - Indicator of mechanical sympathy and control over their vehicle
Car setup/feeback/Car development - You can be the fastest man in the world, but if you cannot set up the car, you wont get anywhere!
Likeability/Media capability - Arguable here, but if a driver is not liked, it can reduce his options, and lower his potential achievments - Although a counter is that you might get overlooked for not being ruthless enough!
Wet driving capability - Shows up poor car control, and how brave a driver really is!
Dealing with adversity - Bouncing back from bad luck and poor results to perform again quickly
Even this is not an exhaustive list, for sure, but we cannot rely purely on WDC wins to decide who is best!
As a side note, I always find it odd how little the Prof gets talked about when comparing great drivers, perhaps some sort of indication in itself. Amazing when you look at his record of WDC wins and consistancy over his career.