This is such a cruel, cruel question for a bookworm. I don't actually think I can answer this one because it's usually "the book I just read" haha but! I'll try.
Haroun and the Sea of Stories by Salman Rushdie. Full disclosure, I read this in high school as part of one of my English classes and I don't actually remember much about this book beyond I really enjoyed it and it's been on my re-read list ever since. But considering there are very few books that I loved enough to even think about reading again...? I think Haroun deserves a spot purely for that. I'd love to experience it again and see how it holds up now that I'm an adult.
It by Stephen King. What a long, laborious book; there's some kind of poetic metaphor about it being as heavy to lug around as the grief and fear inside it. Well worth it, though. This was the first book by King that I ever read and I fell in love with his style instantly.
Nineteen Seventy-Seven or Nineteen Eighty-Three by David Peace. I still can't in good faith recommend the Red Riding Quartet to most people--they were some of the hardest books for me to read in terms of content, never mind the unusual writing style that made it a little hard to follow--but almost six months later, I'm still entranced with... whatever the heck Peace was doing lol. Seventy-Seven and Eighty-Three were two of the more surreal books in the series, yet despite having no idea what was going on half the time, they were two of the most compelling because of that surrealism.
The Great Gatsby by F. Scott Fitzgerald. What is there to say about this book that hasn't already been said over and over again? I love Fitzgerald's style, and the complex and tragic relationships between characters in The Great Gatsby are some of my favorites of any novel I've read.
The Magician's Nephew or The Lion, the Witch, and the Wardrobe by C.S. Lewis. They're just very nostalgic and comforting in general. You have to have your cozy books, you know?
There's actually one other book that I would've put on this list as number 5, but I'm not allowed to talk about it yet as I won it in a GoodReads giveaway and it isn't being sold yet. 🤫 All I can say is that it's a dark psychological thriller that I have strong feelings about (good and bad) that, despite a borderline offensive ending, is somehow still a book I think I would recommend to fellow horror fans. Once it's officially published either at the end of this year or the start of the next, I'll write up a proper review for it here.
(And, perhaps unpopular as I didn't see too many people talking about enjoying it, Disappearing Act by our Mr. Sheehan should also be on my top-something books. I really loved "Skin", "Rose", "Desert Donkey Legs", and "House in the Country"; they all show a lot of promise for Rob as an author. I really hope that he releases another book with more of the same!)