For me, personally, this goes to Fallout 4. I installed the game 3 times thus far, and the furthest I got before uninstalling in frustration was 10 hours in. There is pretty much nothing I like about that game. The interface is like pulling teeth, visual style is boring, writing is terrible, pacing is shit (seriously, power armor and a deathclaw fight right at the start? who thought that was a good idea?), crafting is tedious, settlement building is just terribly implemented. I couldn't find a single thing about the game that I enjoyed. I loved both the original games and F3/NV, but this was just awful... I know, quite a few people disagree with me here and I respect that. But I just can't bear FO4.
I would like to list the latest Heroes of Might & Magic game here, but I honestly wasn't expecting much since the last one was such a mess. What did surprise me was that this one was a mess in completely new ways - instead of it being an inherently interesting and well presented game brought down by a few shoddy mechanics and a terrible forced online component, this one was unburdened by the online crap, but was instead simply a crappy game overall.
Another nod would be to Age of Decadence. I kept an eye on this over the years, hoping it might turn out good. And you know what, maybe it did. I just couldn't bear to wade through the atrocious presentation, horrid visuals (CAN you make a game more brown?) and most of all, the clunky, horrendous interface. I'm blaming this on my age - as I grow older I'm less inclined to wade through such a mucky exterior to maybe find a diamond inside.