I'm old enough to be a keen follower of the Clarke/Asimov axis so Foundation, Rama, 2001/2010/2063/3000 and the Robots series of short stories are always near the top of my list.
However for sheer "hard sci-fi" my single favourite book (not part of a series or trilogy) is "The Two Faces of Tomorrow" by James Patrick Hogan. In 1979, he foresaw fully automated computer control, the prospect of AI and the internet, in a way much closer to the actuality of today that most other sci-fi authors.
For humour, I cannot pass up the quirky, sometimes brilliant Terry Pratchett's "Discworld" series