You can't overlook the fact that today's game is tailored for someone like Harden. Specifically, the fact that teams switch so readily on pick and rolls means that all Houston has to do is run a pick and roll with whatever player is being guarded by the opposing team's weakest player on defense, often a big because clearly most bigs struggle to guard out on the perimeter. This makes iso ball far simpler for a guard. Also, no hand checking and even minor contact counts as fouls these days.
Imagine if Wilt, Jordan, etc. had a mismatch every possession and had the freedom to do whatever the heck they wanted. Harden is an awesome offensive player, but greatest ever? Pfft.