I've spent a lot of time on the trails in the Huron National Forest and the surrounding areas, Glennie, Mio, Barton City, Rose City, etc. Lots of nice, well maintained trails there.
I've also spent some time in the Mancelona and Kalkaska areas. This area tends to get quite a bit more snow and usually has snow when the others don't. Lots more straight-aways along power line a pipeline runs. But IMO the trails in this area take a bit more abuse and tend to me more bumpy.
has some really helpful maps for snowmobiling in MI.