Well, best is rather subjective - it's all about personal preference. With that in mind, the only way to measure "best" is as "most popular", which is not that good a measure.

For the UK, here's the top 21:
1. The Lord of the Rings, JRR Tolkien
2. Pride and Prejudice, Jane Austen
3. His Dark Materials, Philip Pullman
4. The Hitchhiker's Guide to the Galaxy, Douglas Adams
5. Harry Potter and the Goblet of Fire, JK Rowling
6. To Kill a Mockingbird, Harper Lee
7. Winnie the Pooh, AA Milne
8. Nineteen Eighty-Four, George Orwell
9. The Lion, the Witch and the Wardrobe, CS Lewis
10. Jane Eyre, Charlotte Brontë
11. Catch-22, Joseph Heller
12. Wuthering Heights, Emily Brontë
13. Birdsong, Sebastian Faulks
14. Rebecca, Daphne du Maurier
15. The Catcher in the Rye, JD Salinger
16. The Wind in the Willows, Kenneth Grahame
17. Great Expectations, Charles Dickens
18. Little Women, Louisa May Alcott
19. Captain Corelli's Mandolin, Louis de Bernieres
20. War and Peace, Leo Tolstoy
21. Gone with the Wind, Margaret Mitchell
http://www.bbc.co.uk/arts/bigread/vote/

This link:
http://www.guardian.co.uk/world/2002/may/08/books.booksnews
gives the top 100 books from a world audience, with Don Quixote as the top book, but no other rankings given. This was from 2002, so hopefully avoids the JKRowling farce :-)

My favourite book is not on either of those lists - so don't expect to find your favourite there, too.

Most critics would probably choose something like Ulysses by James Joyce, or Moby Dick by Herman Melville, or The Ambassadors by Henry James, or The Magic Mountain by Thomas Mann, or Middlemarch by George Eliot, or something by Dickens (Bleak House, probably) or Anna Karenina by Tolstoy or The Brothers Karamazov by Dostoyevsksy.

Don Quixote by Cervantes might well win out - the trouble with a lot of the truly great novels is that a lot of them are quite difficult to read. Foreign language ones might be a better bet than English ones, because the translation will be in more accessible language.