Nockamixon State Park
Nockamixon is killer. I live in Eastern PA and it is far and away my favorite course. Is it very challenging and lushly beautiful. I would highly recommend making the trip. Also, I would wager the course is at least as difficult as any you have played.
Tips on Nocky:
1. Bring PLENTY of water. If it's hot out, bring 2x PLENTY. No joke. I have dehydrated myself on this course with 3 full liters of starting supply.
2. If you can, wear long hiking pants. Much of the rough is really rough, and there's a better than good chance you will need to go into it to retrieve your discs.
3. This is a long and grueling course. It takes me 2.5 to 3 hours to complete with a group of 3.
4. Prepare for a great time! I routinely finish 30 over par here, and frequently I notch an eight or two. The terrain is so beautiful, the atmosphere so serene, and the golf so challenging that my score doesn't seem to have an effect on me.
If you do come out and are looking to make a weekend of it, about 30 minutes north of Nockamixon you will find Blue Mountain's Skyline Park. It's 5 bucks to play, but it's as challenging as you are likely to find anywhere.