My list of hard but I would love to play everyday:
Merion (because 3 courses in one)
Oakmont (because post-tree removal, the tee shots and almost every shot is incredibly fun since you never know what is going to happen and still a mix of short holes)
Pinehurst No. 2 (because hard to lose balls)
Baltusrol Upper (similar to Oakmont, the shots are fun)
Honors Course (a little different, but I've only played match play. And I had a blast. Query: Match play may make "hard" courses manageable)
Nanea (because Hawaii but not lava golf)