Most of my choices have been named already, but I think a couple have been missed.
Hal Clement (Harry Clement Stubbs) – Mission of Gravity is a classic, but so is Needle (from which they ripped off The Hidden, without even a thank you). Also Iceworld, Cycle of Fire, and others. There’s a sequel to Mission of Gravity (Starlight) and to Needle (Through the Eye of a Needle) that are often neglected. All of his hard sf is good. He also wrote some historical novels, that are hard to come by.
Larry Niven – great hard sf, sometimes in collaboration with others.
Jerry Pournelle – a Niven collaborator, also a great hard sf author in his own right. I was peripherally involved in something that became a story of his.
James Hogan – I liked his earlier stuff (Inherit the Stars and its sequels, for instance), but his later stuff gets weird.
Robert L. Forward – A former Hughes Research Lab star, later founded his own company. Dragon’s Egg, his first, has already been mentioned. Try also Rocheworld/The Flight of the Dragonfly. TimeMaster is an attempt to treat time travel on a scientific basis.
Jules Verne – I know that a lot of his stuff looks quaint, but for a guy who wrote over a century ago, a lot of it stands up. His lapses are more interesting for what they reveal about the history of technology and human lack of foresight, than to his individual shortcomings (why didn’t the Nautilus have a periscope or torpedos? Obvious, I suggest, only in hindsight.). The Weapon of Destruction featured Guided Missiles. The Barsac Mission/City ibn the Sahara was the first time anyone called for help over the radio (in fiction or real life). Carpathian Castle and ** Diary of a Newsman** featured televisions. Tribulations of a Chinaman had the Chinese hero using rubber survival suits at sea. Robur the Conqueror had a heavier-than-air flying machine that was built not of metal, but of composites! I could go on. Verne’s status as a hard SF writer is too often overlooked.
Frederick Pohl I’m surprised his name doesn’t come up more often in discussions of hard sf, but Pohl obviously does careful research before setting pen to paper.
Arthur C. Clarke – Too obvious to go on about. Heck, the man invented the communications satellite.
Isaac Asimov – Mentioned above. Sometimes his use is trite, but he always educates.
** Robert A. Heinlein** – I might have missed it, but I don’t think I saw his name in the thread above. The perfect blend of hard science and characterization, even if you disagree with his politics. The man could handle the difficult feat of explanation without seeming pedantic. Read damned near anything he wrote.
L. Sprague de Camp – less known for his sf than his fantasy or his nonfiction, but always a careful researcher.
There are others I’ll remember only after I close this, I’m sure.