Staal. As great as Mac Truck was last year, I don't think he was as good as Staal was two years ago. So at their best (so far), I think Marc has the edge. And then even last year, with Staal at 75% (?), there was nobody I was more comfortable with having on the blue line than Marc come playoff time. He was beastly and was one of the few Rangers that elevated his game when it mattered most. That's significant, and not mentioned very often.
Long term, I think Staal/McDonaugh are going to be symbiotic, with Staal shutting down a team's top player who's a power guy, and McDonagh shutting down the speed ones. Like if Ovechkin was still Ovechkin, Staal's pair would be the #1 pairing for the night. Facing, I don't know, Phil Kessel, Mac's pair has #1 duty instead.
I love Girardi, but I think he's a distant 3rd. His ice time is the highest, because he's a RH shot, and has nobody eating into his icetime. Staal/Mac eat into each other's, and even still, are logging insane amounts of time.