The numbers support Visnovsky being the best dman this season.
If you think +/- defines defensive play. To me, Pronger's 24+ mpg and 200+ hits to go along with a +/- that is still in the 20s is more impressive.
I do agree that Visnovsky has had a great season, and I would easily put him 2nd, but I think the +/- is boosted by the sick lineup he plays with.
I am sure that the debate will be on....but remember on this team Joe Thornton is only a +6. Makes Visnovsky's numbers more amazing.
Also, when you think of it, the guy that has won 5 of the last 6 Norris Trophies in the NHL, has a similar game to what Visnovsky has shown in the BRHL2....although Prongs did win one back in 2000. No one says Lidstrom has great numbers because he plays with Stevie Y and Fedorov and Zetterberg and Datsyuk and some of the best defensive players in the last decade.
Depending on what criteria you go by, either Pronger or Visnovsky would be a great candidate, but you can't discount Visnovsky's year. (And Xtrastats would give the trophy to Visnovsky... )