A sci-fi website that I look at regularly asked the question, whilst reviewing reprints of old pulp SF, do science fiction classics still matter? It's the sort of question that comes up frequently - if you've got speculative fiction based on an imaginary future, does it lose its power once that future fails to come true or once modern technology has outpaced the writer's ideas to such an extent that what was once futuristic is now dated and anachronistic? Well, I tend to agree to with the article which stated that, for the most part, a well-written story still holds it power regardless of when it first came out (the example they use being Jane Austen - would you dismiss Pride And Prejudice simply because it was written in a different time with different values to our own?*).
I'd go a little bit further than they do, however. I'd say that there are very few examples of science fiction that becomes worthless or irrelevant simply because it no longer presents a vision of the future. As has been said before, the majority of science fiction is an extrapolation and reflection of the period during which it was written and, as such, even the most pulpy old SF can, seemingly paradoxically, show us something about our past.
So basically, sci fi is aces even if it's rubbish because, even then, it's still brills. With that overly long preamble in mind, let's highlight some classic works that don't always get a look in with those who tend to shy away from the science-y fiction-y stuff.
The Forever War by Joe Haldeman - A realistic view of war in space in which, due to relativity, soldiers fighting for weeks or years at a time return home to find centuries have passed. leaving them increasingly alienated and disconnected from the war they're fighting. First published in 1974, it's a clear allegory on America's involvement in Vietnam - Haldeman was a Vietnam vet who was wounded in combat.
Last And First Men by Olaf Stapledon - A history book which spans the next couple of million of years of human evolution. It's unusual in that it is written as a faux text book, although don't let that put you off - the breadth of speculative imagination on display is fantastic for a book published in 1930.
The Stars My Destination (or Tiger! Tiger!) by Alfred Bester - Bester is often cited by a lot of sci-fi/fantasy/horror authors as one of the greats. The novel features a genuinely unlikeable, monstrous protagonist but is no less compelling for it.
I could go on and list many more - including some truly pulpy ones Edgar Rice Burroughs' Mars books** - but I've rambled enough already and, to be honest, I don't have as snappy an ending as I did a beginning. So I'll just leave it there for now.
* Personally, I'd dismiss it because I found it tedious and annoying but that's just my opinion.
** And please note that I don't mean pulpy in a dismissive or disrespectful way - I enjoy those books as much as the ones listed above, just perhaps in a slightly different way.