Well, for the almost full season of riding that I've been in northern chicago, I've tried a fair amount of trails. Lots of little preserve runs around town, but the wife and I always finding us heading north for new and exciting gnar.
Greenbush, in the northern kettle moraine forest is something that is absolutely noteworthy. Of everything i've ridden within 2hr or so of me, this system is FANtastic. I was so excited, I forgot to take pictures! Its a nice distance loop of very flowy, well cut and thought out single track. It was in very good shape as well. Its definitely worth the drive if you are trying to get away on a saturday or something to go see new and exciting single track!http://goo.gl/maps/qD8z