Almost anything by Vernor Vinge. "A Deepness in the Sky" in particular is amazing. I was/am particularly impress by how he handles the idea of fundamental limits on the scope of human achievement, and by how he weaves that into the story so that it's there at all levels. "Diaspora" by Greg Egan has a great story, huge ideas, and probably the most cogent explanation of sentience/sapience ever written. "The Diamond Age, or A Young Lady's Illustrated Primer" and "Accelerando" have already been mentioned. "Reteif!" Made me laugh out loud. Larry Niven's "Destiny's Road" points out the difficulty of interstellar colonization. "Ringworld" blew my mind. Gene Wolfe's "The Book of the New Syn" taught me to pay attention; "Dirk Gently's Holistic Detective Agency" by Douglas Adams reinforced the lesson. There are a lot of good ones; I could go on for ages...
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