Bonucci is one of the worst I've seen on "one on one". If the player is running at him with the ball, you can be sure that it's either a foul in dangerous position or the opponent will be the winner of the duel.
Without fixing such an important aspect of defender's game, he won't be a top CB.
P.S: Of course, if we do create a team that defends so well that it won't allow players to freely run at CBs (in counters, for example) than Bonucci's handicap will be masked, but right now we're far from creating such a team.