The Three-Body Problem by Cixin Liu, and the other two in the trilogy too. Hard, but accessible. I've never read a series so dense with cool and believable ideas that are unlike much I had come across before.
The Reality Dysfunction by Pete F. Hamilton (and the rest of the series). If you want something a bit more traditional in form. More sex, violence and so on.
Ach, loads of others. Roadside Picnic really stuck with me too. Though might not be what most people would call 'hard sci-fi', not sure.