Why so many votes for Hitchhiker's Guide?
A lot of Adams's concepts are pretty much impervious to becoming dated, plus(/because) it's funny.
I voted for the four that aren't monuments to the preachiest impulses of mid-century liberalism, although
Star Trek at its best is really good too and I wouldn't vote for
Doctor Who if I weren't taking its distant past into account.
My
actual answer is still the Hainish Cycle, even though the more I know about Le Guin's creative process the less I want to know.
A Canticle for Leibowitz would stand as a near-perfect self-contained gem if
Saint Leibowitz and the Wild Horse Woman hadn't been so badly bungled (although I'll maintain as long as I live that
Wild Horse Woman had good ideas that it just didn't execute well).