The dude has made some unreal films.. E.T, Jaws, Jurassic Park (stole the job from Jim), Raiders, Saving Private Ryan, Schindlers List. Then some not as popular ones which I loved like War of the Worlds and A.I.
His ability to pump out decent films over decades consistently is what sets him apart, I don't doubt that Jim could have done something similar but it's just not the route he wanted or chose.
The only objective measure of greatness though is $$$, after 4 more Avatars, Jim will be the highest grossing director of all time, averaging 3x more $$$ per film than Spielberg.