asimov's best novels are his most slept on:
the end of eternity
the gods themselves
check those out if you haven't
arthur c. clarke: childhood's end
stephen baxter's xelee sequence is dope and is really as "hard" as hard sci-fi gets, but the concepts and scope are epic, try the novel flux, about the miniature inhabitants of a neutron star...pretty killer
http://en.wikipedia.org/wiki/Xeelee_Sequence