Tim Duncan was absolutely unstoppable during his prime. That, plus the 5 rings and longevity has him at no. 1 without a doubt.
Dirk could score from all over the court. He was basically a taller, less mobile/athletic, Kevin Durant but with a much better post/mid-range game. In his prime, no one was stopping him. He's at number 2.
Garnett was the best defensive power forward in the game during his prime. True enforcer who was money from mid-range and the post-up, who could get into the head of the opponent by his sheer physicality and trash talking ability. Had he been healthy the Celtics would have repeated in 2009. They came within 5 points of another ring in 2010, and one game within another finals appearance in 2012 when his squad lost to the Heat in game 7 of the ECF. (I believe they would have beaten OKC had they advanced to the finals). Had reality been just a teeny bit different, KG may have had 4 rings right now, as opposed to only one. He's at number 3.
Barkley was a bulldozer and one of the most underrated players in NBA history. He's at number 4.
Malone had a fantastic career. He's at number 5.