It feels like we should now be able to more or less say what the top consistently hardest individual puzzles are. I made a shortlist of ten potential contenders just going by my rough estimate of questions I remember seeing in this thread (in no particular order):
Mountain floor panel
Shipwreck vault
Double RGB panels in container
First red tetris panel/Green tetris panel on quarry door
Colored stars on town door
Last of the six synchronized panels in the mountain
Mountaintop floor panel
Container lock panel in town
Beach vault
Color gradient panel in cave
Does that sound right?
About half of those I completely agree with and are my hardest, here's some I'd take out for me personally:
Container lock panel in town, container double RBG, quarry tetris panel, colored star door in town, possibly even Mountain floor puzzle
Replace with:
Blue & yellow mountain air bridge paths, one or two of the cylinders at the bottom of the mountain (forget which), tinted yellow Town door (with stars and tetris), parallel Tetris in cave