I live in St. Louis and have thought of mixing the Mississippi River Trail (MRT) with the Grand Illinois Trail (GIT). The GIT runs around the upper third of the state and goes through the Chicago area.
Thats my original neck of the woods. I'd try something close to the Illinois and Miss rivers. Everything else is going to be somewhat monotonous. If you are looking for mtbike trails, then Jubilee College SP near Peoria is one of my favorite places to ride in Illinois. If you have the time, check out Shawnee NF south of Carbondale. Great roads, great trails.