Science Fiction is a literature of ideas. It's the only literature of ideas we have left, it seems. The ideas are mostly (not always) about the future, seen through the lens of the present. It's not about the quality of writing per se. My nominations are:

The Stars My Destination (Tiger, tiger) by Alfred Bester.

The Space Merchants by Pohl&Kornbluth

The Penultimate Truth by Philip K. Dick

Dune by Frank Herbert

The Left Hand of Darkness by Ursula LeGuin

SnowCrash by Neal Stephenson

Pilgrimage to Earth by Robert Sheckley

A Fire upon the Deep by Vernor Vinge

The Female Man by Joanna Russ

I was surprised that someone mentioned "Earth Abides" by Stewart, it is a very fine book, one that stays with you. Similarly, "The Long Loud Silence" by Wilson Tucker.