Spraguebrook Park in Sothern Erie Co. has a nice trail that follows the contour of a ravine system. Hunters Creek Park is also another favorite near East Aurora. Iroquois Wildlife refuge can be a blast during spring bird migrations.
There are also some good trails around Ellicottville.
The Western NY Mt Bike Association (WNYMBA) is a good resource for local trail conditions. They also do pretty much all the maintenance on trails around here.