By Isaac Asimov. Some softer elements, like FTL and antigrav cars, but otherwise a rational series.

By James P. Hogan. This 5-novel series is noteworthy because of the history it establishes for the solar system and is a reaction to 2001: A Space Odyssey.

By Vernor Vinge. The first book in the Zones of Thought series and tells the tale of a galactic war. In the story the mind's potential is determined by its location in space.

By Larry Niven. Niven's Known Space universe contains a hard science approach to science and the engineering of giant space structures, but does require unobtanium.

By Robert A. Heinlein. A vivid look at what it's like to be in a future military, starting from bootcamp. Definitely part of the Military Sci Fi sub-genre. The bugs are a bit silly, but there are some strong 'hard' elements that make up the story.

By Joe Haldeman. The story of soldiers fighting an interstellar war against aliens. Includes some interesting fallout to relativistic travel and the result of time dilation.

By John C. Wright This trilogy has some features of hard sci fi and uses nanotechnology well. It also depicts a far future society that is plausible. It is a more literary work than is typical of the sub-genre.

By Bruce Sterling. While the avian-like aliens are implausible the story itself shows humanity moving in different directions, which is far more realistic than a uniform society often depicted in the softer sci fi.

By Stephen Baxter. A complictated sequence of stories and novels that span several billion years and includes the expansion of humanity and wars with aliens. The science is believable the world building is stellar.

By Fred Hoyle. This book incorporates many sciences in an encounter with an alien intelligence that takes the form of a cloud of gas.