Hard for me to judge since I know almost nothing about NFL.
But just for perspective. NBA is by far the more global sport. Football may be America's no.1. But I'd bet basketball is crowdsourcing it's athletes from a dedicated fanbase that's at least triple the size.
And only ~450 world's best basketball athletes end up in the NBA. As opposed to how many NFL athletes?
So I'd have to guess NBA is more likely to have collected world's best athletes for its particular sport.
That said, "its particular sport" is the key here.
I don't know what's the criteria for an "athlete" here.
Are 7 ft guys like Yao good athletes? Certainly in basketball; speed and coordination is amazing, and that's what is relevant. Probably not in football. Just like some of those football fat butt guys wouldn't be considered good athletes in most other sports.