When I think of toughest 3 caches come to mind. Bald Mountain and Tumbledown Challenge but those were tough because Shewolf and I did them with ice on the trails. We were climbing up frozen waterfalls both days.

The other one is Swamp Monster in Mass. I don't recommend that one to anybody. At least not by foot like I did it.

I have done some others that were tough but those three stand out.