Who are the best expert of KID opening?

Bronstein of course. Turned it from a cruddy, cramped sideline into a creative, attacking force. Also, a nice man. Here's Seirawan talking about him https://www.youtube.com/watch?v=SAymAaV92qk

Many posts many answers, not accurate question.
Best player ever to regulary play KID: Probably Naka, only see Fischer and Kasparov competing.
Bronstein might be the one who improved theory the most.

I once played in an Open with Bronstein also competing. He was so modest, happy to play against anyone and he didn't seem to care if he won or lost. All he cared about was whether any beautiful or interesting ideas appeared in a game.

Bronstein might be the one who improved theory the most.
Bronstein was a great player (and person, a rare combination indeed), but it would be unfair to give him the whole of the credit for making the KID a mainline opening.
It was three young Ukranians (actually all Jewish ;) in the late 40s: Bronstein, Geller and Stein.
Theory-wise, Geller and Gligiric were the two top GM contributors.
Hikaru Nakamura and Ding Liren