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.
2666 by Roberto Bolaño, it took me 2 years to finish. I read it more as a series than one continuous book because I needed breaks. It was so good, just bleak and depressing. Def read a lot of comic books and fantasy in between to help break it up, it baffles me that some people can just do it all in one go!
Started 2666 in my teens and finished it…not in my teens.