I don't see why people keep saying Dillon. Just because he hits and is in the middle of scrums doesn't mean he's a leader. He wasn't exactly one for Texas iirc. Sure he brings grit but he tends to do silly stuff at times.
I say right now Whitney. They did bring him in to give some leadership and he seems willing to take that role if he hasn't already.
If not that, then go the three A's route with Benn, Eriksson, and Robidas and then see who takes over for the rest of the season and the off season.