I'd like to give a shout out to Julian and my fellow commentators at EuropeanCivilWar.com.

While people can nitpick, every book you chose is a great book, but I would actually place one book above Dune as the greatest SF book ever written.

The Mote in Gods Eye

On their own, Niven and Pournelle write damn good books. Niven especially with a sense of biology driving behavior, and Pournelle with his extensive study and research and among other things, psychology ( read through the essays in A Step Further Out at some point) and history. Along with Drakes slammer books using Greek legend and history as their starting point, basing many of the Falkenberg stories on actual history gave me the impetus to look at history again after having it beaten out of me by the drudgery of "schooling"

The two together were fucking awe inspiring (and Oath of Fealty holds up damn well despite improvements in tech since then).

While Mote may not have quite the level of internal philosophizing of Dune or quite as strong a set of characters – which, yes, I also love – it is one of the best executed first contact novels ever written with a truly unique alien race, where events remorselessly follow the logic of the situation. It encompasses biology, psychology, anthropology, sociology, warfare, engineering, and religion.