Clear winner has to be 1984 - not just the best SF (and it is SF), but probably the best and most important novel of the 20th century.
.... But keeping to the tradition of "the best" meaning as many good ones as I see fit, just like with the vintage cars, lets add some Ballard - unusually for SF a genuinely great writer - At the very least will want the Drowned World, and perhaps high Rise, and I think the latter just about qualifies as SF.
Aldiss, perhaps not such a "great writer" as Ballard, but pretty good all the same, and what an imagination. Let's have Non-stop, and hothouse for starters.
Keith Roberts' Pavanne has already had a mention
I confess to a soft spot for (some) Heinlein - on his day, he's actually a good writer, albeit in a very simple easy-reading style, though has written some dross as well. i think Glory Road is perhaps my favourite. It's a mistake to label him a fascist; he's more a "right wing libertarian" or even a "right wing anarchist" - rather than authoritarian: a political breed almost unknown this side of the pond - hence we tend to think of him as Fascistic at first glance.
Philip K Dick - i like Man in the high castle - but confess to not having read Androids dream of electric sheep yet.
For someone more or less drug-addled and / or loopy, he's turned out a stagering quanity of pretty good work. nearly every sci film that's actually any good will be based on a Dick story - eg blade runner, total recall, pay-cheque etc etc.
Athough i liked (early) Asimov as a kid, it really doesn't stand up well on re-reading, so he gets bounced out I'm afraid.
And for the booby price, the truly awful ee doc smith. As a kid I was hoodwinked into purchasing a couple by those wonderfull Chris Foss spaceship illustrations on the covers - they were poor even when I was 11, and utter dross now.
Ah well