Mass Effect 3 is up there. I played 1 and 2 back to back in preparation for 3, and playing it just after that felt like I was punched in the face. The Star Kid is the easy thing to pick on, but the whole writing for the whole game was bad. Plot, dialogue, characters, descriptions, backgrounds, quests, everything. The jarring drops in quality were so obvious to me that I could notice them as they were happening, and the admittedly improved and even fun gameplay couldn't make me stop from thinking how poorly planned and executed the story lines and how badly written the dialogue lines were, and how phony was most of the voice acting. Garrus and Tali spoke like they were self-aware and doing an impression of their former characters, Wrex spoke like he was mentally challenged, Mordin did a 360 and decided to cure the genophage because uhh we had to tie up loose ends with the Krogan. And then there's the sidequests with serious implications that you never see unfold on-screen (resurrecting fucking dinosaurs), and the characters from side-material showing up (Kai Leng), and the massively story important characters and content being DLC and having no impact on the story (Javik, the Leviathan). There's a lot to say, but I'll stop there, you get the point. Also they made Miranda's ass flatter and smaller than it was in 2, and that's terrible.