I would say you have to rule out tips/seams because then it is just a question of what a crack is. Same with non-continuous cracks and flares.

So for splitter cracks that are fingerlocks or bigger:

Yea, loose fingers is probably the hardest (not much feet) with ringlocks a close second (slightly better feet).

I find the hardest is OW is just below knee size. Too big for fist jams, but you can't get your knee in.

There is another tough size just below squeeze chimney (too small to get the hip in). Too big for stacks and too big for knee locks. Awkward size for feet.

Too lazy to look the grade up, but Owl Roof in Yosemite looks like it would be a pretty mean OW.