To date, Keith. No question in my mind. Dude has been an absolute workhorse and a core part of the defense (not to mention almost single-handedly willing the team to a Cup in 2015). I'd argue that he's been our most valuable player of this era without a second's hesitation.
Going forward, Kane. The mileage has finally caught up to Keith, so that's enough to let probably the #2 or #3 guy critical to our success to overtake him.