Good guesses by everyone, however Medill90 was the only one to answer correctly - it's JVZ. It's not related to the last 4 games that he has missed though, as prior to those he was still the only member with a negative +/-, though Taphorn, Lindsey and Ash were close. I'll give partial credit to those who guessed Taphorn though, as prior to the injury, Tap had a worse defensive PPP (1.17) while having a positive +/-.
Demps is below average, although with a caveat that he's been on the floor for 89% of possible minutes, so it's tough to determine how much he is responsible for defending numbers.
On the other hand, to address Rick's guess, Falzon has some of the best defensive numbers of anyone on the team. Lineups with him in the game have only given up 0.91 PPP.
If we only look at the five toughest games we've played (Columbia, UNC, Mizzou, VT, DePaul), Falzon shines again where the rest of the team does not. The team was a collective +7 and gave up 1.07 PPP, but lineups with Falzon were +33 and only gave up 0.99 PPP. Lineups with Olah were similarly good, at +22 and 1.00 PPP. At the other end, lineups with Lindsey were -24 and gave up 1.08 PPP, while lineups with Taphorn gave up an astounding 1.27 PPP.
While it's great that we've been able to blow out some of our opponents, data like this concerns me for the B1G season. Let's hope that the team is riding high on confidence that helps us keep winning these tight games, because the competition is going to stiffen real quick.