I don't buy a lot of bad games. And the ones I DO buy, usually end up getting sold off not long after I buy them.
So I basically only can go off memories, since unlike my library of GOOD games on my shelf, I don't have a box to look at to remind me.
So most of mine are fairly recent, but I REALLY regretted the following:
Mortyr: A sci-fi, WWII themed shooter from the late 90s, NOTHING about it was redeeming. Terrible graphics, sound, gameplay, level design, AI, everything. Just a miserable game that was unbearably bad, even for its time.
Silent Hunter 5: Dumbed-down, buggy piece of crap game, with intrusive DRM. Three strikes right there. The other SH games had their issues, but were otherwise excellent games. SH 5 was utter garbage from the start.
Sherlock Holmes and the Awakened: Sounds like a great premise; Sherlock Holmes investigates Cthluhu. Starts out alright, then takes a nose-dive. The puzzles are terrible, dialogue is terrible, graphics and sound are uninspired, and worst of all, IT NEVER MENTIONS CTHULHU BY NAME, ONCE! It's like those old sports games that couldn't get the license to the pro sports they were trying to represent? Yeah, it felt like that. Just terrible.
Black and White: Another game that started out so promising, but took a nose dive in a hurry. I liked it for the first 5 hours, then you realized just how flawed it was. I'm sorry, but when you (as a GOD), have to individually pick up villagers and place them with the opposite sex because they want children (because...you know...how else does reproduction happen in the animal kingdom??), you know there's a problem with the game design.
Lord of the Rings Conquest: Basically, a horrible Battlefield knock-off, in the LotR universe. Nothing more to be said. Just awful all-around.
Master of Orion 3: Already been mentioned, but it would have been a less than mediocre game even without the MOO name attached. With it, it's terribly disappointing.
Far Cry 2: Similar to above. The first was amazing in so many ways. the sequel dragged the name through the mud, almost literally. I've been playing FPS since shareware Wolfenstein 3D, and it gave me motion sickness worse than any FPS I can recall.
Ghost Recon Advanced Warfigter: Purchased at launch, and it had some of THE WORST friendly AI I have EVER seen in a game. Period. PC Gamer even joked your squad "Put the SPECIAL in Special Forces". Just atrocious. The game also had other bugs, wasn't nearly as deep or as fun as the real GR games, and ran like a dog on hardware of the time.
Civ 5: Playing the series since the first game. Loved them all, in their own way. I hated the changes to the game they made. Of all the games listed here, it's probably the "best", as in its good in its own right, but it's also probably my biggest disappointment as a gamer, and forever dashed my hope for the series.