Heinlen is hard to beat, pretty much across the board. Though one could quibble that once he hit the 80's or so the sci fi component went downhill.

His juveniles are some of my favorite books I read as a kid (Well that and Dr. Doolitlle and Freddy the Pig Detective).

Larry Niven, Ringworld and the Integral Trees are REALLY tough to beat in terms of sheer scope.

The Escape Pod podcast occasionally has space-related stories. Here's a short one that remains one of my favorites for its dark humor: Off Base, by Stevens R. Miller

I'd have to say that Hyperion and the full Cantos by Dan Simmons has it all. In the end it's all about the story, so if the science doesn't have a good story behind it, it can fall flat. Dan's work isn't has 'hard' as some but is still weaved in very well.

Can't wait for the movie!

Gateway is a good series to throw into the mix.