You fell in love with a game and it's characters, sunk hundreds, maybe even thousands of hours into it. It became a comforting, immensely satisfying part of your daily life. Then you heard a sequel was coming and got really hyped but when it came out it was utter rubbish...
Honestly, its gotta be Oblivion. Its really close between it and Morrowind, but I strongly disliked the way you have to get information in Morrowind for the story. I dont mind the reading at all, it's the fact that you have to ask every n'wah about every detail to get any info that, while realistic, I ultimately just found to be too tedious.