Man, this is tough. There are lots of ways you can measure bad. I could go with some games that seemed unfinished and lacking playtesting like WarWizard or Ruins of Myth Drannor. Or something that was just bland and disappointing, like Oblivion or Fallout 3. I guess I'll go with a mix of games that were poorly designed and too disappointing.
1. Ultima 9. Man, this game broke me. I was a huge fanboy of the Ultima series. I even completed Ultima 8 and didn't think it was that bad (nor good, tbh). I thought Origin would be around forever. I gave up on this after the starting area, and that's after a few attempts at getting the game running at all.
2. Dragon Age: Origins. I'm not trying to be edgy here, but this game was a big disappointment after going through the Gold Box games. It was made a couple of decades after Pools of Radiance and by a company known for RPGs, yet somehow the developers couldn't figure out how to make town guards respond to bandits running around in their town or a combat system that was actually tactically sound. And the story was cringe all around. I finally got the hint in the Deep Roads and stopped playing BioWare games after this.
3. Outer Worlds. I played the first two Borderlands games and they are alright for time wasters. I walked into this game not expecting much, and was still shocked at the poor quality. I don't know how you fuck up cloning Borderlands' simplistic story and gameplay, but Obsidian managed it while also making it several degrees uglier. Round of applause to all of you boys at Obsidian... you truly have a gift for crap.
4. Final Fantasy 12. I used to like the Final Fantasy series, but this was the last one I played. There were some misses like Final Fantasy X (though FF X-2 was a guilty pleasure for the job system and campy story), but this is the first game that added the crime of being boring. Square Enix should have kept it to the four characters (Princess, soldier trying to clear his name, Han Solo knock off with sexed up sidekick) and dropped the two annoying tweens. I did recently pick up the Zodiac Age version since I'm a sucker for jRPGs with a job system, but stopped again after the first hour. The game is just that fucking boring.
5. Neverwinter Nights. I do appreciate what they were trying to do here. The developers were trying to create a replacement to the Baldur's Gate and Icewind Dale series with a new game engine that would allow for people to create their own modules and run them for friends. And they also incorporated the new D&D 3rd edition rules. They got the modulation part right along with the new ruleset, but the engine made gameplay cumbersome, slow, and graphically ugly. Again, I appreciate the effort but it still fell short.