Foundation

Isaac Asimov's "Foundation" is aptly named. It is the foundational SF novel in many respects. It may have been the first SF work to engage in world-building, big ideas, and speculation of the future on this scale and in this combination. Asimov is one of the fathers, if not the father, of science fiction, and "Foundation" shows why.

All future epic, world-building science fiction can be traced to Asimov's Galactic Empire of this series.

Also, it contains one of the best ideas an SF author has ever explored: a branch of science ("psychohistory") that can predict the future. This knowledge predicts bad things, and thus necessitates an attempt to conserve whatever they can of their current world's knowledge to serve as the 'Foundation' for the future. It's almost as if a massive Wikipedia were created for the purpose of gathering all the knowledge in the universe we went into a collective dark age so that we would have a library of knowledge to use when we'd reemerge into a renaissance.

It gets complicated in some parts- a few too many characters to keep track of, too many moving parts that detract from its great ideas- but overall, a very compelling, interesting foundational novel in the history of science fiction.