For this year, Karlsson then Weber. He's awesome but I just find it hard to give it to Weber considering he's got another potential nominee as a defensive partner with a top 3 goalie in the league behind him... His individual contribution to winning games doesn't feel as large as Karlsson's.
Going forward long term, assuming no injuries, I'd say it's a tossup between Letang/Karlsson/Pietrangelo. Of course, I'd like to have any of the players listed in the poll on my team...
Erik Karlsson is fantastic offensively, but I have seen him play pretty badly in his own end and while neither Chara or Weber are as good offensively as he has been this year, he is nowhere near the defensive player those guys are.
I just get the feeling that in the playoffs, whoever the Sens play, is going to try and key on Karlsson and hit him as much as they can. They have to catch him first though. I am looking forward to see what he does in the playoffs, but I would take Chara and Weber over him in that order. Everyone else is after those 3.