He's just a beast. I personally think he was the best CB in the last WC. And his defending style reminds me of Nesta at his peak.
I'd argue he was the best defender overall, yes. He only played half the games at centerback, but then his performance in the final was terrific. His jobs at fullback are often a bit underrated. Even though he had terrific defensive displays there as well, including twice keeping Ronaldo from scoring at tournaments (Euro 2012 and World Cup 2014). I also remember an outstanding performance at leftback, when he was still at Hamburg. When Robben was destroying Bundesliga defenses for fun in 2009/10, Boateng kept him in his pocket in Hamburg's 1-0 win, easily Robben's worst game that season. He's so quick and can easily keep up with tricky and pacy wingers, which makes him such a fantastic overall defender.
He was always incredibly talented, outshone Hummels and Höwedes (to be fair to Hummels, he played DM, next to Khedira and behind Özil) in the u21 Euro win in 2009 (with the wonderful 4-0 final win against England). Since he moved to Bayern, he got more and more mature, added consistency to his game and often stepped up in big games for Bayern and Germany. He's definitely a huge reason along with Neuer, why we broke every defensive record in the league.
And he's only 26, like I said, his peak years as a defender should still be ahead of him. He's definitely still improving. Since Guardiola took over, his ability on the ball reached a whole new level and his reading of the game got better and better. It's a joy to have him play for Bayern and Germany, wouldn't trade him for any other defender in the world.