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...
For me Just Cause 4 is the worst one for too many reasons to list. Also, I'm still not sure about Kingdom Come: Deliverance 2, but that could have a lot to do with not being ready to play a middle aged, yolked Henry lol
(Edit: Oh and Sniper Elite 6 seems like a contender too. Although SE5 was very poorly received at the beginning until they fixed the issues and then it turned into one of the most awesome games ever so who knows...)