This year I discovered the Elderling Saga, by Robin Hobb. Starts with the Farseer Trilogy, Assassin’s Apprentice. Great epic fantasy.
As for sci-fi Isaac Asimov’s Foundation is great.
Neuromancer is also a great read, and one of the earliest examples of cyberpunk.
Since you asked for a single book, and you might not be keen on jumping into multi-book sagas, I’d say go for Neuromancer
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