Well, I can see two ways of interpreting and answering this question.
1. Was he the best defensively? As in, did he "almost always" hang back, take very little risk, and prioritize not allowing the other team to score. Maybe not as much as another "purer, stay-at-home defenseman."
2. Was he the best defensively? As in, if he wanted to stop you, could he? Then yes, probably.
It's like when people ask who the best goal scorers are/were. I like to answer Gretzky and Lemieux, more because of the way I interpret Question 2. Yes, these two were balanced offensive giants who could rack up tons of assists as well, but if they both wanted to just concentrate on scoring goals, getting 100 in a season would not have been out of the question for them.