No doubt that you’ve already had this suggestion, but “Dune” is probably the finest piece of Space Opera in print. The only problem is that it tends towards fantasy more than SO. But Herbert very subtly builds up the world in which the story takes place without resorting to any exposition.

And, of course, the new Battlestar Galactica, although the newer episodes are turning into a regular soap opera. Enough with the strained marriages already! Can we have some more Cylon religion and Space dogfights please?

the 3 books are titled:
Out of the Silent Planet (set on Mars)
http://en.wikipedia.org/wiki/Out_of_the_Silent_Planet
Perelandra (set on Venus)
http://en.wikipedia.org/wiki/Perelandra
That Hideous Strength (set on Earth)
http://en.wikipedia.org/wiki/That_Hideous_Strength

Note that the individual book links contain big spoilers…

{from Wiki}:
Space Trilogy

Main article: Space Trilogy

His Space Trilogy or Ransom Trilogy novels (also called the Cosmic Trilogy) dealt with what Lewis saw as the then-current dehumanizing trends in modern science fiction. The first book, Out of the Silent Planet, was apparently written following a conversation with his friend J. R. R. Tolkien about these trends; Lewis agreed to write a “space travel” story and Tolkien a “time travel” one. Tolkien’s story, “The Lost Road”, a tale connecting his Middle-earth mythology and the modern world, was never completed. Lewis’s character of Ransom is based in part on Tolkien, a fact that Tolkien himself alludes to in his Letters of J. R. R. Tolkien. The last novel in the Trilogy also contains numerous references to Tolkien’s fictional universe, and can be seen partially as a homage to Tolkien. The minor character Jules, from That Hideous Strength, is an obvious caricature of H. G. Wells. Many of the ideas presented in the books, particularly in That Hideous Strength, are dramatizations of arguments made more formally in Lewis’s The Abolition of Man.

Another science fiction novel, The Dark Tower, was begun, but remained unfinished; it is not clear whether it was intended as part of the same series as the completed novels. The manuscript was eventually published in 1977, though controversy arose about its authenticity (see the article on The Dark Tower (1977 novel)).