1. The DaVinci Code
2. Angels & Demons
3. Deception Point
4. Digital Fortress

Having now read Brown's complete opus, this is the order I would come up with if asked to rate the books by how good I think they are. That also seems to be the general consensus, at least according to Amazon sales ranks, which are in the same order.

The list is also almost (but not quite) a reverse chronology of the novel's release dates, which could indicate that Brown is getting better at what is doing from book to book.

On the other hand, the list also reflects the order in which I read them, so maybe I am getting bored here. In fact, it seems like the main building blocks for characters and plot are always the same: You have a male and a female protagonist who are somewhat romantically involved and share sheets in the final chapter, a powerful secret society, and a villain who pulls strings in the background, who sends an outlandish assassin after our heroes, and whose identity is finally revealed to be the supposedly trust-worthy friend in high places, as opposed to the obvious malefactor that is presented earlier.