MOST DIFFICULT: Finnegans Wake by James Joyce
We could be talking about pretty much any James Joyce novel here (they’re all wonderfully nonsensical), but let’s talk about Finnegans Wake because I personally feel that it’s double the difficulty of Ulysses.
If you wanted to, you could (and probably should) read Dubliners, A Portrait of the Artist as a Young Man, and Ulysses to help prepare you for this, but I’ll tell you this right now: you’re still going to struggle. You’re going to have no clue what’s happening. Scholars have spent DECADES trying to come to some sort of consensus as to what this godforsaken book is actually about, and they have yet to succeed. The plot is nonlinear, the characters fluid, the text largely unreadable owing to Joyce’s idiosyncratic language and stream-of-consciousness writing style.
For these reasons (and also about a billion others), it is the most difficult novel you'll ever have to read for school, and if your teacher assigns it, they probably hate you.