Impossible, unanswerable, I’ll try to give you a short list of some of my top fiction books though. Presented in no particular order:

Paradise Lost (Milton), Frankenstein (Mary Shelley), The Iliad* (Homer), The Double*  and The Brothers Karamazov* (Dostoevsky), The Master and Margarita* (Mikhail Bulgakov), Odd John (Olaf Stapledon), The Picture of Dorian Gray (Oscar Wilde), Good Omens (Neil Gaiman & Terry Pratchett), Ulysses (Joyce), Infinite Jest (David Foster Wallace), The Sorrows of Young Werther (Goethe), the entirety of The Divine Comedy* (Dante Alighieri), Orlando (Virginia Woolf), The Trial and The Metamorphosis (Kafka)

Asterisks denote books which I’ve only read in translation. This is also excluding plays, comics, or other miscellaneous texts that aren’t really books (e.g. Socratic dialogues, poetry volumes, short stories, and so forth), for the record.