There are too many to have a single favorite. By far the coolest one I've ever ridden is the
Colchesters Causeway Park . You ride across the opening of Mallett's Bay on Lake Champlain. For a large section of it you are in the middle of a lake!
Best one in Colorado so far is the Rio Grande Trail from Aspen to Glenwood Springs. Ride the bus from Glenwood to Aspen and then enjoy 40 miles of (fast) downhill.