Many many factors make a driver a GREAT.
Speed alone only makes him the one of the fastest, but not one of the greatest. (though i agree Senna is one of the greats)
I usually refrain from Greatest EVER, cos we can't compare fairly.
F1 may be more commercial nowadays, but still, only the best would stand above the rest.
A Great driver must impress on many counts. Results/Achievements alone may not be the absolute, but it's a strong representative. A great driver may not get many WDCs/records (ie Gilles), but a driver who achieved alot must be great (Prost,Schumacher,Fangio).
Of course, speed and skills are on track attributes that we judge the driver. They are what we fans can judge for ourselves. The abilities to push the car beyond it's capabilities, the car control, the ability to not throw the car out of a race or overstep its capabilities, the skill to set up the car to achieve the best, the ability to drive around problems are all great attributes of a great driver.
"He has no competition" is just a lousy excuse used to brush off a driver's achievement. Afterall it's easier than to admit that a driver the critics dislike to be superior to the rest..
