Top 5 for me (in no particular order):
The Handmaid's Tale - Margaret Atwood
Brave New World - Aldous Huxley
Lord of the Flies - William Golding
A Long way Down - Nick Hornby
Harry Potter series - J.K. Rowling
Not because they are literary masterpieces (although I think they are all great), but just because I found them very entertaining and/or thought provoking without feeling like a slog to read.
How could I forget Lord of the Flies on my list.
That one is a masterpiece, even if Harry Potter is not.
Have you ever noticed that in that entire book there is not one superfluous sentence, not even one superfluous word?
Every single word in there is to advance the plot.
A lot of writers make long descriptive paragraphs about how blue the skies are and how pretty the butterflies, or what happened to someone ten years ago that explains what they are doing now, but in Lord of the Flies there is zero fluff.
Beautiful and direct language, and not one word that can be removed.