I’ve already mentioned Asimov, whose most important works I’ve read, excepting his Robot series. He published over 500 books in his lifetime, however, so I’ll never really be done Asimov. When I see a second-hand copy of an out-of-print story or essay collection, I’ll always pick it up. But what about the other two writers comprising the “Big Three of Science Fiction”?
I think I can equally say I’ve read Sir Arthur C. Clarke’s most important stuff. The entire Rama series (the original works as a stand-alone but the later sequels are also minor gems, I think), the entire Space Odyssey series (which is solid enough, but of course the original, 2001, is the only must-read), one of his most well-known stand-alones, Childhood’s End.
Missing from that list is The City and the Stars, The Fountains of Paradise, and I’d like to read his last published book, co-written with Frederik Pohl, The Last Theorem. It started with Clarke’s outline but he got so sick that Pohl pretty much did all the heavy lifting himself. He finished a final draft, Clarke took a look and was happy with it, and died days later. So all three of those are on my list.
Robert A. Heinlein is possibly less prolific than Clarke, and certainly much less so than Asimov. But I have the most catching up to do with him. I’m definitely considering reading every single one of his novels, which is probably why I’m much further behind than the other two.
His work is generally broken into early-, middle-, and late-period Heinlein. Early Heinlein includes the Golden-Age short stories in Astounding and other magazines (a good chunk of which I’ve read), and his juvenile novels for Scribner (plus a few non-juveniles written under pen-names during the same period). I’ve read about half of the juveniles, leaving perhaps five or six to go. His last book of this period is Starship Troopers, and also the most influential, but there’s a lot of good stuff in this period and it’s worth reading almost all of it.
Middle-period Heinlein includes what are generally considered his greatest works: The Moon is a Harsh Mistress and Stranger in a Strange Land were both written during this period of the 1960s and early 1970s. I plan to read all of these (excepting possibly his fantasy novel, Glory Road), but having read his two most important works, Moon and Stranger, the ones I really need to read next are Farnham’s Freehold and Time Enough for Love.
After a period of ill health, Heinlein began writing again in the ’80s. These last few novels generally aren’t considered his best work, tending to meander and proselytize a bit on his political beliefs. That said, while his plotting was less tight, some of those flashes of brilliance that make Heinlein great still occurred, at least by some accounts.
I haven’t read any late Heinlein, but plan on reading at least The Cat Who Walks Through Walls, and we’ll see where I go from there. Job: A Comedy of Justice was, apparently, flawed, but also important, so we’ll see.
I should note, though, there have been far more than three important SF writers the last 70 years. I’m familiar with Pohl but not Bester, Silverberg and Haldeman but not Niven or Ellison, Bujold but not LeGuin. There’s still a lot of catching up to do.