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EDIT-I hadn't re-read the OP since it had "voting rules" added to it, I'm not officially voting for anyone since I don't own a team
this is just my opinion on the matter for whatever it's worth...
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I'll admit I'm a little biased since I play on the same team as him but my vote for defensive MVP is Adam Avalanche.
As a DT he leads our defense in:
Tackles: 72 (19 more then the next guy on our team)
Hurries: 65 (15 more then the next guy in the league!)
Sacks: 23 (7 more then the next guy on our team and only 2 of those sacks came against the 3 worse teams in our conference, there are a lot of quality sacks there)
Tackles for Loss: 29 (18 more then the next guy on our team, this stat isn't tracked on the league leaders page so I can't tell where that stands in the entire league but I'm going to go out on a limb here and say it's pretty good.)
Forced Fumbles: 3 (Tied for team lead)
All that on a defense that has allowed 1.2 ypg (game) rushing, 0 rushing TDs and also leads the league in pass defense and fewest passing TDs.
There are certainly other good choices, LastDinosaur being one that jumps to mind but Avalanche can dominate games as a DT which is a position you rarely see that kind of impact from.