If you basically know that everyone will live up to their potential then James is obviously #1, but I don't get why Duncan wouldn't be #2 on everyone's list. He is the greatest PF in league history and was an integral part (if not the most important part) of 5 separate title teams spanning nearly 20 years. His teams won 50 games every single season of his career, except the 50 game season, and that includes a 66 game season. The Spurs won 70% or better of their games 7 times during a span of 9 seasons took a 3 season respite and then reeled off 4 more 70% or better season, before a bad season of 67%, and his final season at 81.7%.
There is no right or wrong answer here and it is mostly fun driven.
I get what you are saying, we know how their career played out and order can be made more easily. But imagine this for an example:
swap Kobe and T-Mac. I know they are different draft classes, but stay with me.
T-Mac gets to LA, Shaq is there already molesting the whole league - 3 titles, boom, they don't fight like cat and mouse - boom 5 titles. T-Mamba mentality, best wing since Jordan...
Meanwhile Kobe is in Toronto, gets to a lesser basketball environment and no Diesel there of course. Team results are lacking in his first 4/5 years, maybe an All-star game or two, probably not as east had some shoe-ins at his position. He is having his own ideas of how things should be run, gets into a dispute with management (sounds familiar), trade happens and it all goes in a much different direction.
Or what if Duncan didn't have Pop by his side? Would he still be great? Yea, sure. 5 titles through 2 decades on one team? Not that probable. Pop gave him a huge boost and stability, perhaps more than any other coach would. What if he went to the worse medical staff in the league? Or if he is being coached by a guy that "logically" thinks he should always play 42 MPG as he is the best player on the floor? You want your best players on the floor.
There are million roads. Ping pong ball here, ping pong ball there - history would rewrite careers easily.