Larry Niven is one of my favorites when it comes to classic hard sci-fi. His Known Space stories encompass quite a bit, Ringworld and its sequels likely being the best known. Niven's collaborations with Jerry Pournelle can be somewhat more hit-or-miss, but A Mote in God's Eye and its sequel The Gripping Hand are both good reads, as is their unrelated novel Footfall.
David Brin's Uplift Universe is also fairly engaging. The first three novels (Sundiver, Startide Rising and The Uplift War) are standalones, and the last three together form an extended sequel to Startide Rising. Perhaps not the hardest sci-fi out there, but the cultural basis for the civilizations is quite fascinating. A weird diversity of alien species abounds.
“In our every deliberation, we must consider the impact of our decisions on the next seven generations.”
-- Great Law of the Iroquois Confederacy