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.
Dracula and Frankenstein, because they just seemed like something all real readers should get around to at some point.
Mary Shelleys Frankenstein was amazing! And a way easier read than Alexandre Dumas Count of Monti Cristo, Man in the Iron Mask and Three Musketeers. Those are the only ones i read. I know he wrote more and lots from his attempt as a lawyer. (I spell badly! Please forgive any errors)