Frank Herbert's always good--
, of course, but also some of his short stories and other novels.
Easy to see parallels between the Dune
series and current events, a la 'spice' = 'oil', Arrakis = Arabia (or the general, oil rich regions of the middle east, etc. etc.
Along the same lines: Asimov's Foundation
series is good: has a sort of elite, behind the scenes aspect (hope that's not a spoiler!) that looks a bit different to me now, after reading a lot here at R.I., than it originally did when I read it 10 or 20 years ago.
Also: Joe Haldeman's Forever War
and Heinlein's Starship Troopers
--but read Heinlein's S.Ts. first.
Last (for now) but not least: John Birmingham's Axis of Time
alternate history series. ( http://en.wikipedia.org/wiki/Axis_of_Time
--just a few I've enjoyed; maybe they're obvious, but if you haven't read 'em yet, you're in for a treat.