The Minoqua area (Vilas county) is the greatest place to ride in northern Wis and Upper Michigan, in my opinion.
However, dirt bikes and quads are not welcome there. I've ridden there since I was a kid, when the logging and fire trails belonged to everyone.
Now, mostly because the snowmobile organizations maintain the trails, and think they own them (and the DNR not arguing the fact, because they are subsidised by the fees and the free maintenance), they are closed to dirt bikes and quads. The State Forests there had reorganization plans recently, and I was involved in that process, but they still voted dirt bikes and quads out. There is also some private land that touch the two-tracks, and these owners will only allow snowmobiles and hikers. (Don't tell anyone, but I still ride there and feel like native American Indian robbed of his rights--but I stay out of sight of everyone.)
Iron County, to the west and right next store, is almost completely open to dirt riding. But it does not have the beautiful trails and the many lakes of Vilas County.
I used to ride the Nicolet and the Cheq. too. But in most places, I don't think you're welcome there either. Lately I've been riding the fringes of the Nicolet on some private land and paper company land. I've been staying out of the Nicolet. Heard of riders getting big fines.