I have been meaning to come back to this since reading Selfish Gene, but haven't gotten to it! I suppose I should :)

Dawkins is not an original researcher. He is wonderful at synthesizing research and writing eloquent popular books.

The importance of The Selfish Gene is to write a correct summary of the neo-Darwinian project. In the future (perhaps even now), it will be more famous for the coining of meme (in an appendix IIRC).

However, The Extended Phenotype is a remarkably original exposition of important (then) contemporary frontiers of Darwinism. It is a powerful idea, powerfully described.

It is certainly his most important original contribution to the literature. And a beautiful book. True and convincing. It will change your view of the world. There is no higher praise.