There's no law preventing any of you from going ahead and saying that Leonard is the best player in the league. If you guys think he's better than Curry and Lebron, then go right ahead and announce it to the world. In a couple of years when he's the best player in the league you can tell everyone how smart you were.
When he hasn't been the best player in the league, Duncan has been called the best power forward, which is a qualification. In his prime, Duncan was actually the best player in the league and he didn't need a throng of teenage girls on a message board to defend his honor by dissecting every compliment in print about him to find the hidden insult.
The fact is, Leonard is not the best player in the league right now yet he has a few prominent people, like Barkley, saying that he is. He's getting a lot of credit for being a key part of a Spurs defense that's full of guys playing great defense. If he's really that good, and if you really believe that he's going to get better, then all you have to do is wait around for it to happen.