One of the best I've found is The Forever War by Joe Haldeman. The physics of time dilation is not only interesting, but crucial to the story.
It would like being on a Nuclear sub, without a war.
Very boring.
Yeah, that definition of classic is a bit grey, but I don't think I would include Haldeman as classic SF and, though I love his books, I'm afraid I certainly wouldn't include Reynolds as his first book wasn't published until 2000. Incidentally I've just recently finished House of Suns and that is just brilliant space opera.
That's not to say he doesn't do a excellent job of portraying plausible space travelOkay. In the words of Emily Litella (Gilda Radner), "Never mind".
Actually I think that's exactly why I prefer (sacrilege) more recent writers; as you say they tend to have much more plausible settings (not just the spacecraft); for me much of the classic SF, particularly space opera, might be well written and have great characters and plots but the science tends to be just a little too wild and unlikely. There was a sort of naïve enthusiasm with which many (though not all) often allowed themselves to be carried away. But then, who knows, maybe in 50 years time the current crop of writers will be looked back on as being equally wildly implausible.Oh, I'm not taking umbrage. It's just that most of the Golden Age authors played fast and loose with physical science. Just push a button and the latest quark drive gets you where you want to go in a week or less (sometimes just in time for dinner). I'm thinking of the rigors of actual travel between sometimes unthinkably distant points in the universe. Some of our more recent writers tend to be more realistic. Niven wrote "Mote" in the 1970s. I'm also thinking of C. J. Cherryh in both her Alliance/Union and Chanur tales, many of which were written in the early 1980s. I found her descriptions compelling. But it's hard for me to think of many SF space travel tales written any earlier which give me the same sense. I'm sure there are some that just haven't been named yet.
Vance is a peerless describer of planets and cultures, but in the majority of cases, Vancean space travel is a fairly cursory excuse to get from A to B.Simak, Cherryh, Vance...