I go with KG, Duncan is the better leader, but KG has been a PF for his whole career (except maybe some SF time back in the day?)
McHale was phenomenal offensively and defensively under rated. But VERY short peak. When saying greatest of all time - you have to consider how many great years there were.
I can see the argument the other way Duncan, but I think Duncan's titles have too much influence on that perspective - which argue more for rating Popovich over McHale on the GM totem pole.
The thing I like about KG is he can play next to anyone - I'm not sure the same is true for Duncan.
Malone and Barkley are great, but I'm not sure they have the same two way impact as the three above. And I don't really know any of the old school guys, but I have a hard time believing any of the old timers could actually play in the modern NBA.