The Reasoning for Kawhi Leonard:
Kawhi Leonard is the 3rd youngest NBA Final's MVP in NBA history, behind two legends with Magic Johnson and Tim Duncan the only younger players to win the Final's MVP. Soon Duncan and Ginobli will retire and Leonard will have the reins to this San Antonio team. He has a very complete offensive, he still needs to hone some of the moves to a more perfect way. He could stop a train on defense, as he could stop even the best players in the NBA.
The Reasoning for James Harden:
James Harden is already a star which makes me think that he has already hit his ceiling because he has an insane offensive game while he doesn't show any interest in playing defense. Once he ditched the 6th man role with the Thunder, he shone offensively while he defensive shortcomings got magnified.
The Reason for Giannis Antetokuompo:
He is the least developed of any player here and that can cause some concern. He is an abysmal situation with Milwaukee because they are trying to turn him into a point guard when he should be a small forward. If they ditch head coach Jason Kidd, he could raise his ceiling because at point guard, his ceiling plummets.
The Verdict: Kawhi Leonard
Kawhi Leonard is the most proved prospect out of all of these while still having the highest ceiling. He is a two-way force and his nickname "The Hand" is very accurate, he has the biggest hands of anyone to ever start an NBA game. He's got next in line for NBA stardom.