No real and definitive definition of greatness, or highest level.
A course can be great for many different reasons. A course can be great for different levels of golfers. If anything comes close to a definition of proper challenge, it might be a course where it is possible, but not easy, for a golfer to shoot within a few strokes of their average score. Not too hard, not too easy. (i.e., a course where normal set up would make a tour pro shoot 78 isn't great). But, how warm is the perfect bowl of porridge? And does it get there by being cooked to hot and allowed to cool, or taken off the stove just at that right temperature?
The real answer to this question is to further subdivide the various lists into courses ranked primarily by fun, character, beauty, perhaps intrigue (or temptation), and challenge.
But, you knew that already!