Oh that one? It's Super Hexagon. [WARNING:will probably make your brain spin]



You have to last 60 seconds to complete a level, and as shown, 'Hardestestest' is the highest difficulty in the game (the easiest level starts with difficulty 'hard', by the way).
Long story short, I was only 0.72 second away from beating the entire game. So close, yet so far ;A;