Really good stuff, both epic in scale and fairly hard scifi.
The Ringworld Series is pretty interesting.
Rendezvous with Rama is a classic.
Standing on Zanzibar for something a bit more highbrow.
The Neuromancer Trilogy for that Cyberpunk edge (or the Bridge Trilogy which is a whole vibe)
The Cryptonomicon, although that might not be considered scifi?
Snow Crash is pretty fun too
