For me it's Diablo II. Granted I've played my fair share of D2 since launch, and also recently on a private server with a comrade from hexbear, but I still feel like years later the game didn't grab me as much as D1 did.
Granted I don't hate D2, but for a game that I keep coming back to, D1 takes the prize.
The Sly Cooper series. I just didn't like Sly 4.
I was ok with them tweaking the gameplay a bit, as there was a huge change from sly 1 to sly 2. But some of it never sat well with me, like some of the art and UI seemed off (although I loved the animated cutscenes, they were very well-put-together and felt like I was watching an actual cartoon on TV).
But what really condemned it was the story, they left it on a cliffhanger and then decided to never touch it ever since and it's been more than a decade. They should have just made Sly 4 complete the entire story.