I think I have said this before in another thread - the Quabbin Reservoir in Massachusetts. The watershed area around it is an unofficial state park/nature reserve. For me, the best time is during a prolonged drought (bad for Boston) and the water level is real low, the shore is populated with huge rocks and boulders the size of a humvee. I still would like to explore the northern end and eastern shores.
Because the water level can vary quite a bit from season to season, the photo opportunities change on a constant basis.