Fallout 3 wins my medal. It got so bad that I stopped playing the game because of them. I constantly got stuck in world geometry and the AI of NPC's became infuriating when "allies" would start attacking me for no reason whatsoever. I do wish I could have explored the game more, but the bugs just pushed me away.
Another, more recent game that had bugs kill it was the new Medal of Honor. There's nothing worst than a strongly trigger-based game (by trigger-based I mean an NPC has to do something to trigger the game advancing) that doesn't work right.
The final straw came in the mission where I was teamed up with the guy on the cover box who reminds me of Opie from Sons of Anarchy but not as cool. Anyways it's a stealth mission where you are supposed to take follow a group of Taliban soldiers and pick off various numbers of them as they break off, thus making sure you don't alert the entire crew. The problem with this is that the game would glitch out and either they would not move in the group like they were supposed to (meaning if you killed one, 10 more would come out and turn your body into swiss cheese) or they would suddenly all smell the American Apple Pie on your breath and kill you in .4 of a second. On my last excursion, I was in cover with Opie waiting the group out, and Opie said that it was all clear. Before the word "clear" could come out of his mouth my player had turned into cooked hamburger, after which he then said, "Get down!"
That's not even mentioning all the broken NPC teammate stuff I saw, like teammates literally running in place and being teleported to other parts of the map, and then having to shoot them to get them out of the running animation so the game can finally continue. It really shocks me that such a buggy game was released by EA, especially with so much hype and hope riding on it. I'd rather go play the original MOH for PS1 than play the reboot again.