These are actually in order of non-preference:
Ultima 8: Pagan. What the fuck is this shit? Here's a game with NO redeeming features whatsoever. None. The only good thing about this game is that the engine was later used to make the Crusader games.
Ultima 9: Ascension. I'll just quote Spoony: "BETRAYAL!" Also, bugs ahoy. But unlike Ultima 8, it has a few redeeming qualities, such as looking awesome for a game its age, and sporting an awesome soundtrack.
Descent To Undermountain. Such a promising game, such an awesome engine. Such a huge disappointment.
Fallout 3. "Oh hey, I'll pass a stupidly easy Speech check to convince a SUPERCOMPUTER that what it's doing is wrong. Then I'll disarm this nuclear bomb with one hand while I spend 30 minutes sweating over this boobytrapped wooden door on that beat-up shack over there. But first, I'm going to beat my dad unconscious by shooting him in the head."
Gothic 4: Arcania. See my signature.
Gothic 3: Forsaken Gods. It's one of a handful of expansions that I know of that actually makes the game feel worse than it was.
Might & Magic 9. Fortunately for me I never had to play this, but I had to help a friend of mine (and then comfort him) who was trying to play it through. A game so bad it can make a grown man cry. Literally.
Neverwinter Nights OC. It has the feel of something that was slapped together in the wee hours of the morning before being released. Fortunately the expansions are of a better quality.
The Bard's Tale (2004). It's funny, has a few interesting songs, clever bits of dialogue and the voice acting is awesome. Those are its only redeeming qualities. Don't play it, just watch the LP on YouTube instead.
Star Wars: KOTOR. I actually gave up playing this somewhere around the half-way mark. When it isn't boring, it's jumping the shark with backflips.
Captive (PC version only). It's so bugged, it's unplayable. The game doesn't give you enough cash or XP compared to what you'll need to stand up against the monsters thrown at you.
There are other games that I'd love to mention, but it wouldn't be right for me to do so because I haven't played them. Almost every AAA title in the past 10 years that calls itself an RPG, Lionheart, Final Fantasy 8+, overambitious beat'em-ups that try to pass themselves off as RPGs, Lands of Lore 3, Legends of Valour, and many more.
Suggestion to Jaesun: Compile this list as well, and use it to subtract votes from the "Best" list.