For sure the worst game I have ever played is Unlimited SaGa for the PS2. It isn't so much a game as much as it is torture. I would say it feels like a glorified board game, but that would be an insult to board games.
On a slightly different note, the most frustrating game I've ever played was Super Ghouls and Ghosts on the SNES. Quite possibly the hardest game ever, but somehow I managed to beat it once without cheating many years ago.
