Statistically so far it has been:
Diarra, Calcaterra, Alleyne, Jackson, Clingan - Did most of their work against Georgetown, 7 points allowed in 12 defensive possessions. Also held Florida to 3 points in 5 possessions. Smaller/unreliable sample size overall though.
The starting lineup with Alleyne in place of Karaban (Newton, Alleyne, Hawkins, Jackson, Sanogo) has a few more possessions played together and looks quite strong defensively.
My guess for best defensive lineup is the first one I mentioned putting in Hawkins for Calcaterra: Diarra, Alleyne, Hawkins, Jackson, and Clingan. Have only played 2 possessions together all season. But they didn't give up a point, so I'm sticking with that answer lol.