Haven't been here in some time! Must have missed this one way back, but better late than never?
Some of the entries here baffle me. I wouldn't call some of them bad games by any means, maybe not great or lacking in one way or another, but not bad. Even Oblivion, which I hated for some time, did have some somewhat redeeming qualities - in the end it wasn't a bad game, just a mediocre one.
But enough about that, these are the games I feel were absolutely terrible (for me) and irredeemable:
Neverwinter Nights - expectations were so high for this one especially after BG2 but it was one of the dullest games I've ever played, ALL of it, the writing, quests, music, combat, can't think of anything that was good. I will say the 1st expansion was a half-way decent experience and the 2nd expansion was good, but the OC was horrendous.
Dungeon Siege - I put maybe 2 hours into it and then couldn't play it anymore. A complete snorefest.
Ok, that's all I can think of at the moment. I've found other games to be flawed but still enjoyable.