Since I don’t see it yet…

Super Hexagon

It’s one of the simplest games possible, the controls are “clockwise” and “counterclockwise”, and there are no distracting characters, setting, or story.

And yet the easiest level is – quite accurately – labeled as difficulty “Hard”. The next 5 levels (6 total) go way up in difficulty (and labelling) from there.

Each level lasts 60 seconds. If you can survive that long. I’ve never unlocked the final level myself, so I don’t even know what it is like, but I can guess.