Wenger is delighted with the way the Frenchman has repaid his faith.
"Koscielny is one of the best in Premier League, without any hesitation," the manager said. "He has combination of qualities. Good reading of game, good anticipation, great commitment, great speed and [he is] quick on the turn.
"He is good on ball but he has improved a lot in that he now has better control of himself. He is more serene and calm, one of the very best for sure.
"He had a tough spell after [the League Cup final] and last year I took him out of the team for a while. That's where I give him great credit because he came back in convincing way and took being left out in a positive way."