Worst game you've ever played?

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Dirt Racer for the SNES comes to mind. 

It used the Super FX chip to output some admittedly impressive 3D graphics for the system, but it ran at about 15fps, and the controls were mind-bogglingly awful, with the cars handling like shopping trolleys on polished ice. The slightest course correction could see you careen into a wall and spin out, at which point the race was effectively over. Borderline unplayable.