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.
I don’t consider finishing a book an accomplishment, any more than I consider viewing a great painting or sculpture as an accomplishment: the accomplishment is the creation of art, not the consumption. Many of the books mentioned here I’ve read multiple times: Ulysses, Moby Dick, some volumes of In Search of Lost Time, - etc, length does not a masterpiece make: Heart of Darkness is a short work that very much challenges the reader and some of Shakespeare’s sonnets are as challenging as his plays. Art is NOT a competition, consider yourself lucky to have the ability and the circumstances to be able to read great works (but don’t make it about yourself)