So I narrowed it down to four (with two different books tying for third place)
1) The Lord of the Rings by J.R.R. Tolkien
This book (The Lord of the Rings Trilogy was originally one book) is, in my humble opinion, the greatest work of literature ever written. It is epic, it is an adventure and a love story, it is a hero's quest, it is fantasy at the highest level, masterful story-telling and beautiful prose. I very much doubt I will live to see anything eclipse it.
2) Notes from a Small Island by Bill Bryson
If you've ever wondered what it means to be British then this is the book for you. It captures England and the English in hilarious detail. It is pant-wettingly funny, irreverent and so true!
3) Riders by Jilly Cooper
(warning, this is an 18+ book!)
I know this choice will surprise some people but it truly is an outstanding book. Especially reading it from the point of view of an author. It captures the anti-hero to perfection. You hate him and you love him, and you can not put the book down because he fascinates you so completely. Riders intertwines so many stories, each one satisfying in it's own right, and spans about twenty years. I was thinking about it for weeks after reading it (and gobbling up all the other books in the series to plunder the lives of the side characters), and you can't ask for more than that.
3) Harry Potter by J.K. Rowling
Because... well, it's obvious really.
I'd like to add, though, that I know a lot of people who have not read this book and have no desire to do so. Which is fine, that's your choice. But I challenge anyone who has actually read it not to agree that it is a totally brilliant story!
If I were allowed more then I would want to add Tom Sharpe, Douglas Adams and C.S. Lewis in there, perhaps some Agatha Christie and Georgette Heyer. (Yes, it is clear that I read and enjoy across multiple genres), but those above are the ones that stayed with me, which is, I guess, what every writer strives for.
Wow, it was a tough question though!
I'd love to hear yours. Use the comments and don't forget to tell me why you chose them!