I think the main contenders for this spot is either Bill Russell, Wilt Chamberlain or Kareem Abdul-Jabbar.
First off, out of these 3 I rank Russell the lowest because even though he was the best defender of the three, the greatest winner and arguably the best team player, he lacked the 2 way dominance the other two had. In terms of intangibles Russell might be #1, but I place him below Wilt/Kareem.
Now for Wilt vs Kareem it really depends on whether you value prime output or longevity. This is a big debate among fans and there’s no right answer. For example, would you take Robert Parish 21 years of above average play or Bill Walton 2 years of transcendental play? Would you take Bernard King a SF with an incredible unstoppable prime, or Paul Pierce an above average SF with a long career but a not so dominant prime?
Wilt has the most incredible prime out of any player in NBA history, MJ included. He averaged 50 points a game one season, scored 100 points in one time, grabbed 55 rebounds in one game, led the league in assists etc it goes on and on. However he only played 13 seasons.
Kareem along with Duncan, Malone, Stockton, Kobe, Lebron etc has one of the longest careers of exceptional play out of any NBA player but his prime doesn’t quite match Wilt’s. He played 7 more seasons than Wilt did, yet I believe that Wilt would have exceeded almost all of Kareem’s records (except for rings maybe) if he played just as long as Kareem did.
So for me I’m picking Wilt over Kareem because I believe that Wilt was the more talented player, post knee-injury Wilt was able to block prime Kareem’s skyhook and match him one on one and I do believe that if Wilt was in his prime, he would dominate Kareem. But it’s a close choice and I really don’t fault anyone for picking Kareem over Wilt as well as I understand the reasons behind it.