It could be the same question for a defender too, just asking with regards to the Selke. Selke even more so than the Norris is a question of who is the best defensively whereas the Norris is about who is the best player at a specific position.
So as for the question in the title, is it relative corsi? Relative corsi + taking into consideration zone starts, qual comp, etc?
For example, what stat tells me that Bergeron is the best defensive forward in the NHL (assuming he is, just using as an example)?