The worst game I've ever played? Not just my least favourite, but the actual worst?
Something old and terrible, probably. I remember some SNES games being ferociously awful. There was a Toy Story game for the SNES that sucked so, so hard.....also I remember a Track & Field-type sports game on the PSOne that was just completely unbeatable, totally broken, carpal tunnel-inducing nonsense.
Wait, I've got it. Vampire Rain on the XB360. I hope no one here had the misfortune of playing it. That game was an unbelievable piece of shit.