I would say in terms of story and characters the first book is the best. I found the second and especially third to be far more conceptual and existential. That's not to say that there isn't a story and character development and that it's not compelling, they're just very different feeling and a whole lot darker.

I loved them all, with my least favorite being the 2nd but it was close, and the 1st the best. I wrote a little thing somewhere about them, but basically I find it a beautiful representation of how changing material conditions can entirely flip interests and entire societal structures through the changes due to all the sci-fi shit going on. And you are treated as a knowledgable person in the world with the amount of info you have, but you are limited exactly like humanity as a whole is and the series develops around those limits.