For me, no question it is North Boundary near Pittsburgh....
I live in Buffalo, NY but have done a lot of travel golf over the last 3 years, including playing 8 of the top 25 courses rated here (Harmony Bends still on top with Iron Hill a close second,)...
I have spent 7 disc golf Weekends near Pittsburgh, and Deer Lakes and Moraine are also in my top 5 courses....these are hilly and difficult courses but nothing like North Boundary....playing both of those courses are about equal to one round at NB in terms of the effort it takes...
Crazy elevation gain and loss on the majority of holes...hole #3, #7 and #11 are about as steep uphill as it gets, and there are multiple holes that you must throw over and traverse up and down over huge ravines on flatter holes...
Plus it is a long....really long course...with very few flat lies...
Great design but a brutal workout....not for the casual golfer...