I make no apogies for having The Davinci Code in my top 20.
I know many people like to mock it but as a story it really was an excellent page turner until the very end. I have an interest in hidden signs from the past and have always found the subject intriguing even if not completely convinced of the merit. I didn't go into this book expecting it to be anything more than a work of fiction but Brown managed to keep me on the hook throughout. I enjoy all his books, the secret is to not take them seriously.
Dracula is another great that deserves its place on any must read book list.
Not read the other three, seen the film Tinker Tailor which I thought was OK, but not enough to make me seek out the book.