I read all of his books. Some more than once. Though, most of them are a blur.
The Ancestor's Tale is definately his best book: The most well written, and the most interesting and insightful "tales" (although they are all true). Also, the most fun to read through, despite its length.
The Selfish Gene is his most influential book, and therefore also his most important. Despite its title, it is really a book about the emergence of altruism in the natural world. And, one I would definately recommend!
The God Delusion was probably his "worst" book, but it was also of a very different subject than what he usually writes about. His arguments are well refined, at least.
The Extended Phenotype is very academically oriented. If you must skip over one of his books, it might as well be this one, unless you have an academic reason for reading it.
The Devil's Chaplain is a book of essays he wrote for various reasons, collected into one volume. Also recommeded. This was the first book of his I read.
All of his other books are pretty much a big blur, for me. Most of them were good, but I can't remember anything really distinct about any of the others.
ETA: I do remember that The Greatest Show on Earth had quite a few nice color photograph sections in it.