Probably Don Quixote. It started off really well, but it devolved towards the end into this long-unending self-referential rant full of name-drops and exposition, and I could barely follow any of it and pushing through that was a huge chore.

I later learned I had read a bad translation, and that there is one good translation out there I should try, but the whole thing has left a bad taste in my mouth and I don’t want to go anywhere near that book again.