Knights of The Old Republic 2.
When I first played it, the unfinished nature of the game swarming with bugs and plotholes were the thing that put me right off:
From a gameplay perspective quests could be unfinishable, so many storylines went nowhere, there were clearly areas that were supposed to be used but never got included.
From a content perspective there was the weird retcon to the end of the first game that "Revan flew away with no explanation" and now the Sith have somehow taken over the galaxy despite my Revan purging them and leaving the Republic and Jedi firmly allied and in control.
There was the stupid plot twist of the evil companion you meet in nefarious circumstances at the beginning called Kreia is actually "Darth Traya" I mean, the names are so similar they literally rhyme, and the woman oozes evil and is basically insane, the game could never make it's mind up whether it should tell you straight out or keep it a secret, leading to ridiculous inconsistencies where it was revealed dramatically about 3 or 4 times throughout the game, and each time it would continue on as if your character had no idea, including the final boss battle which was a big plot revelation of "I AM TRAYA!" and even after that at the very end of the game you stand and have a conversation with her about your future like pals, before getting into the spaceship you just crashed in and saw fall down a crevice.
There are several things that are much improved from the first game, and I've heard the actual characterisations are good, and the take on the light/dark side is really smart and different, but I just could never get past the constant inconsistent plot and gameplay points, bugs and messed up quests that plagued that game.