Possibly one of the worst games I ever played was Spawn on the PSX, since I was wise enough to steer clear of Superman 64 and didn't have to endure it's garbage.
On more recent consoles, I'd say Metal Gear Solid 2 & 3 are chief contenders. In fact, the only thing keeping me from breaking the discs is the fact that, storyline/scenes aside, the gameplay isn't that bad. I'd have to say the worst PS2 game I've played/owned would have to be FF12. It's the first FF I've been unable to finish cause I just couldn't keep forcing myself to continue any longer, and that was after about a month of solid playing. I didn't like the main character, the storyline didn't even focus on them most of the time, and the gameplay was gods-damned boring.
Now, there were definitely some crappy NES and SNES games, but particularly in the NES days this was just par for the course. Oh! There was this DS game I can't quite remember the name of it.. the back of the box made it sound like Phoenix Wright-esque game, but when I played it all there was were those gods-damned "hidden stuff in a picture" things that I can't believe people are actually paying to play. I mean, the storyline was interesting, but it had NOTHING to do with the fucking gameplay!
Sorry, it's just.. that's probably the ONLY game I've ever bought, played, and returned within the span of 3 days. I'd almost go so far as to say it's worse than FF12. At least the storyline, characters, and gameplay had SOMETHING to do with one another.