Any work that involves a lot of puns and dialect will be hard to translate, as will anything that uses a lot of rhetorical flair. Puns are often impossible because they involve a "collision" of meaning between two words. If this collision isn't possible in the translated language, the best thing the translator can do is try to come up with their pun. Dialect and slang also will be hard, because slang is kind of a capsule of the culture it comes from. So the translator is going to have to choose a more normal world, losing the original flavor, or pick some slang from the L2, thus bringing an "import" into the world of the work. Rhythm and prosody depend on the harmony between the sounds of the words in the original language. Of course, the words of the translation language will have an entirely different set of sounds, and perhaps the grammar won't even permit similar acts of parallelism. As a stupid example look at

>Vini, Vidi, vici

>I came, I saw, I conquered.

Notice Latin doesn't require the pronoun I. But literally translating "Came,saw, conquered" sounds like it was written by a petulant 14 year old. Secondly, Caesar's clauses are a nice triplet of two syllables. The English version does pretty well there, until it reaches "conquered," which throws the rhythm off. I said it was a stupid example, because the English translates pretty well IMO, but hopefully I demonstrated the difficult work of the translator.

Off the top of my head, I think these would be tough to translate.

-Shakespeare. Too many puns, and the meter is tied to a rhythm natural to English. All poetry is difficult really.

-Huckleberry Finn. Heavy dialect, which often involves the racial prejudices of the time. How can you translate that part? Nothing will be as disrespectful as the word is in English. Furthermore, how do you translate something like: >It must a been close on to one o’clock Do you preserve the grammar error? What's the difference between "close" and "close on"?

-Faulkner. Same as Twain, plus some very heavy rhetoric.

-Joyce. Same as Faulkner, but more puns, heavier rhetoric. If there's anything harder to translate than Finnegans Wake, let me know.

-Gadsby. This book does not contain the letter E! E is the most common vowel in English, but it's not in many other languages. Is the translator "cheating" if constrains the same letter? Amazingly this problem has been tackled in the translation of a different book, La Disparation, a French novel also without any e's either. The English translation, A Void, avoids the character too. However, omitting the E would be too easy in Russian, so that version doesn't contain any O's.