I think it became evident he was the greatest when he retired from basketball to play baseball and golf, returned to the sport and still won three championships. He was the first real 'superstar' in the sport imo. He transcended the game of basketball and became his own brand. He laid the blueprint for the superstars playing today, and he still earns more than all the active players. Greatest on the court and savviest off the court. He was a beast on both sides of the court as well. He could score in the paint at will, mid-range and was a crafty post-player, not to mention a lock down wing. As dominant as he was offensively, he led the league in steals three times.
A lot of these newbies will tell you that he was playing against painters and welders, but he was playing in an era of dominant bigs, and he lived in the paint. There's literally no one in this era who would be able to stop him from getting 30ppgs in a season; especially if one trick ponies like Harden can do it. And beyond all that, he just knew how to win. The only player from this era that comes close to him is LeBron. LeBron in his prime was more versatile, but Jordan's scoring, defense and winning mentality put him over James.