In my opinion, it's Johnson and it's not even close.
His presence in the lane allows a guy like Kramer to play such aggresive defense. Kramar is a good defensive player, but he often gets beat off the dribble and is helped out by Johnson.
He is the most efficient scorer (15 points on 54% shooting), their best rebounder, and the key to their defense (interior presence to coincide with perimeter pressure). He is athletic enough to run the floor and finish at the rim, and he is skilled enough to knock down an open shot.
Hummel and Moore are nice players, but this is a drastically different team without JuJuan Johnson.