In terms of results and talent I strongly believe Lewis is undisputed No.1. But he will never be remembered as the greatest ever due to the 'manufactured' aura surrounding him, the fact that he's got backing from a massive team since childhood.
If instead, he came from the slums and entered F1 in a Virgin racing and kept trumping his pay driver teammate, and then moved up to say Renault, and matched Kubica, and then moved to McLaren and then perform like how he's performing now I'm pretty sure he might have much more of a chance. 
Good point but Lewis is from the slums his dad worked his a$$ off to support his son's love for racing
Just watch the old GP2 highlights...he made fools out of Nelson Piquet Jr., Timo Glock, Lucas Di Grassi, etc. Look what teams they are on now...
Why do you think he went to a top team in his rookie year? Why did McLaren scout and back Lewis since he was a young boy? Maybe McLaren knew Lewis would become one of the best drivers of all time
In terms of greatest ever...Each era was different. A great driver, no matter what era he is born into...will be great in his own right.
I'm a huge basketball fan and people always say Michael Jordan is the greatest of all time
Try asking that same question to my grand dad or my uncle...trust me it will not be Michael Jordan