Is it possible that a brilliant, experienced solver would find the right "tricks" to solve the puzzle? Or is the sudoku such that it can be only solved by some flavor of exhaustive search on the space of potential solutions?
An "easy" sudoku is one solvable by mechanical application of the rules of the game - if there's only number that can be placed in a cell, fill it in, and similarly if there's only one cell in a given row, column, or box that a given number fits in, fill it in there.
As things get harder, other logical steps are required. For example, if there are two cells in a line, and both of them can only accept the numbers 5 and 9, you can conclusively say that no other cell in that line can be a 5 or a 9. That's looking one step ahead, and (depending on how often it's necessary) makes for a medium-difficulty puzzle.
As you move up difficulties things get more complex, and thinking several steps ahead becomes necessary if you want to make meaningful headway.
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I would expect the "hardest" sudoku would not be one that's only solvable by brute force - that's a rather uninteresting puzzle, for a start. Instead, the hardest (in my opinion) would be one that requires making the trickiest insights in order to make headway without resorting to guess-and-check style exhaustive search.
So, a sudoku seems to be extremely hard iff η > 3.
And no, you don't need to do an exhaustive search.
Chokepoints that can only be found in a large set, or having to solve the puzzle with a complicated deduction like an X-wing or Y-wing, raise the number. With guessing as a fully last resort.
After all, if you're looking for a hard Sudoku to solve in that vein, just look at the minimally unique Sudokus. Those required a metric ton of back tracking.