it doesn’t have to be your favorite book or anything. It can be any book that you find yourself thinking of with a sense of pride for having read it.
Personally, I am really proud of myself for not DNFing A Little Life and pushing forward. I read a very good chunk of that book with tears running down my face–mind you, I was reading it on my phone during lectures for the entirety of my first semester last year–and I was always on the verge of putting it down just because of the horrible content. Also, it was pretty long; too long, actually. So when I was done, I was simultaneously Heartbroken, broken (just like in general), and relieved. It was truly a feat.
An honorable mention is A Tale of Two Cities by Charles Dickens, because I swear to God I did not understand a single thing about it even 10 chapters in. Charles Dickens is too much.
A volume of Plato’s complete works published by Hatchett. It’s not that Plato is especially difficult to understand, sentence by sentence (though he definitely can be — eg Sophist, Parmenides, and parts of Timaeus). It’s that he is so discursive it can be hard to connect all the different points he raises. Plus, he also wrote a lot, and some of his dialogues are just really dry (eg Cratylus and Laws).