This is the largest and the best place to ride around Toronto. It costs 20$ a day for motorized vehicles or 100$/yr. It's really multi-use, you will encounter ATVs, trucks, horseback riders. Keep it quiet and be nice! An OFTR membership is not required, although highly recommended.
There's a huge combination of double and single track of varying difficulty. There's also many entry points.
Probably the best way to be introduced to this forest is attending one of the OFTR Trail Rides: several of these trail rides are in the Ganaraska Forest, tens of kilometers and hours of fun, with gas truck and lost riders & found bikes services (beware the sweepers).
The official trail head for motorcycles in the west forest is at the end of the Boundary Road, on the south side - see "W1" on the official map below. If in doubt, go to the Forest Centre, at the Cold Springs Camp Rd & 10th Line.
Tracks:Download to GPS. Or search more routes.