One crude measure I have of measuring greatness is the ability to do something "impossible", such as make low-powered or underperforming machinery perform out of its class.

Some examples:

Fangio - 1957 German Grand Prix - coming from behind and beating everyone.

Moss - the 1961 Monaco win in an underpowered Lotus against the all-conquering Ferraris. Richie Ginther has the fastest lap for that race, but Moss matched the time on the following lap.

Clark - 196? Nurburgring 1000km - demolishing the field in a Lotus sports car running what was basically a Ford Anglia engine. Until the Lotus broke, Clark was leaving them at 30 sec per lap. The Lotus was banned from the Le Mans 24 Hours as being "contrary to the spirit of the regulations"! Also the remarkable performance in the Italian GP in coming from nearly a lap down to lead (shades of Fangio above) and then run out of fuel with the finish in sight.

Stewart - Monaco, 1966? - bringing a Tasman Trophy BRM (much less powerful than a F1) and giving everyone else the fright of their lives.

Senna - the memorable near-win at Monaco in streaming rain in the Toleman - had the race not been stopped, he would have been in with a chance of winning. (The Senna Hagiographic Society would have you believe that the race authorities did this to allow Prost to win. However, even if he'd caught up, Prost would, I'm sure, have made the car JUST wide enough to keep him at bay, as Senna did in the memorable duel with Mansell in the latter's championship year).

Schumacher - that first Ferrari win in the rain in Spain(?) in an evil-handling Ferrari (even Schumacher described it as "difficult") running on 9 cylinders.

But I can never recall anyone making a better overtaking manoeuvre than Mansell did on Senna in Hungary, when Mansell was with Ferrari - as Senna went for a back-marker, Mansell went for Senna and there were three cars abreast on the track. It took Senna completely by surprise and Mansell went on to win.

Anyone think of any other "impossibles"?