It makes me really sad to say this, but it'd have to be Nier: Automata. The first game was a very flawed gem, but won me over with it's tone, cast, plot and music. When NA was announced, it was a super surprise since Nier didn't exactly sell like hotcakes and was more of a cult hit than anything. I remember seeing some art with the green apocalypse vibe, which is totally my jam and then holy shit, Platinum is working on it too?! I don't remember too much from then to when the demo was released but I couldn't download it fast enough. The game felt really nice to play, the music was already vibing with me and the tone (especially at the end of the demo) really got it's hooks in me. I pre-ordered the game and consumed every bit of info and video and forum post that I could. I remember people getting out of bounds in the demo and being able to see a shitty version of the city by doing like punch cancelling and stuff. It was something new! Eventually the game came out...
I took a whole week off work to play it and got a large supply of weed to help immerse myself, like I did with the original. x) It finally unlocked and finished unpacking or whatever at like 2am and I played it till like 6. Went to bed, wake and bake and spent all day playing. It went like this all week and it wasn't long until I got the first ending. Got the second ending and finally the third.
After that I went back to try and get all the side quests done. Got bored with that, so I set about collecting and upgrading all the weapons. Getting all the weapons was neat but grinding for upgrades lost me pretty quickly. Finally I beat up the golden bunny until I was max level, took on the optional bosses and just kinda... stopped. Everything was too easy now that I was max level and had a set of maxed weapons.
Picked up the game about 4 years later, got to the same point (except no bunny grinding) and I still ended up not finishing it fully. I like it, it's a good game! It hit me in the feels many times, the music is wonderful (mostly, some of the nonsense singing gets grating after awhile and it was driving my boyfriend nuts), the combat feels nice and there's lotsa weapon options, great voice acting and characters, thematically solid and the plot is mostly a winner, though some of the decisions were not needed or kinda dumb.
So what's wrong with it exactly? I don't really have anything to say other than I was expecting a life changing experience that would be a favorite game of all time. And what I got was a really good game. I don't think I've ever been so excited for a game before and since. Ultimately, I'd say NA is the better game in most aspects, but the original Nier resonated with me more. I think that probably has to do with the writing for Nier being more surprising for it's time. After Nier I expected NA to be dark and weird, and it was, but it wasn't something that took me by surprise again.
TL: DR - Expectations were unreasonably high and I got knocked down to reality.