I used to evaluate a defenseman's defensive abilities by how few shots or goals were directed against his net while he was on the ice, relative to his teammates. It's become quite obvious over time how unreliable this approach is, because it ignores the quality of competition.
The best defensemen in the league face the toughest opponents, and it doesn't matter who you are, Sidney Crosby and Alexander Ovechkin are going to get more shots and goals than Craig Adams and Boyd Gordon. In fact, for just that reason, the best defensive blueliners will actually allow more shots and goals than the ones facing Adams and Gordon.
To demonstrate, we'll use Gabe Desjardins' Corsi-based Quality of Competition data over the last three seasons. There are 605 player-seasons of defensemen who played at least 40 games, and we'll define two groups, those facing the easiest competition and those facing the toughest.
The Sheltered Group