In the seventies science fiction was getting more political but you wouldn't know it from the Hugo award winning novels in the first half of the decade. The Left Hand of Darkness is the best example of this phenomena which was filling up bookshelves at that point but the rest of the awards went to books that were more about engineering and exploration than humanity.
I suppose that is understandable. The 1970 awards were for books published the same year that the first moon landing occurred and by the 1974 awards a trip to the moon was beginning to look routine. In that context I suppose a return to science fiction's roots with the rest of the awards is understandable even as other elements were developing. Even the New Wave of the late sixties is lost under the this and the winners almost feel like a step back into the fifties.
The most extreme of the science novels would be To Your Scattered Bodies Go which you can make a good argument about it being a sociological novel but my feeling is that cribbing from Lord of the Flies doesn't turn a book with a focus on exploration into a statement on the human condition. A lot of the metaphysical implications of the resurrection of the dead are simply lost in Farmer's story which focuses more on how and why this happened rather than how people deal with it.
Asimov's The Gods Themselves is at its best when he sticks to the chemistry and engineering that drives the first third of the novel. Even the later sections with their change in focus are much better when he sticks to those strengths.
Clarke and Niven are the most blatant about their books being novels of engineering with the stories being completely centered on the exploration of the vast alien devices mentioned in the titles of their novels. Niven's Ringworld does a much better job of giving the exploration some depth than Clarke's Rendezvous With Rama though Clarke's engineering is considerably more realistic (for certain large values of realistic).
All of the winners in this block are big winners with all of them having at least three Hugo wins over their careers (Farmer's third was for "Best New Writer" in the first year that the Hugos were awarded). Le Guin, Clarke, and Asimov notably have won twice for best novel but Asimov's second win was particularly undeserved (I'll deal with that in a month or so). All of them have won at least once for short fiction and if you're willing to extend the award a little for him Clarke even has a victory for dramatic presentation thanks to his script for 2001: A Space Odyssey.
The current scoreboard for all Hugo winning novels reads:
Didn't Like: 7
This time around I enjoyed but wasn't thrilled with The Left Hand of Darkness and Ringworld while the borderline case of The Gods Themselves landed on the "Liked" side of things. My dislike of Rendezvous With Rama was mild compared to the hatred I had for To Your Scattered Bodies Go. The early 70's is the weakest block of Hugo winners until the late 90's when you'll see some venom really start to flow and not let up for a while.