Karlsson has one of the worst goals against per 60 on his team and doesn't play the PK. Yes, he's great offensively, but if he were playing a tighter game defensively, the offensive numbers would drop.
Fox by contrast has one of the best goals against per 60 on his team, and plays over 2 minutes of SH time per game. Dahlin is also on the lower-end for his team in terms of goals against per 60 (not quite as good as Fox), and plays 1:44 of SH time per game.
I'd go with one of Fox or Dahlin. Certainly leading the league in points for defenceman is one of the components of evaluating who should win the Norris and who is the best defenceman, but there's also more to it than just going to nhl.com, sorting by points, and calling it a day. These guys are DEFENCEmen, and Karlsson isn't good defensively.