Best Stand Alone Classic Science Fiction (Nominations Needed)

What is the best stand alone Classic Science Fiction Book (published before 1980)?

We've done best classic fantasy. Best modern fantasy. Best modern Science Fiction. Best science fiction AND fantasy that could have stood alone (but didn't). So now the only category left in this series is stand alone classic science fiction. (We'll start doing series and stuff soon.) As always, remember to take your recommendations to the comments on the blog if you want them counted (not as replies to the posts on FB or other social media.) And though the rules don't change much if you've done a lot of these before, please check them out if you're new or need a refresher.

I spent the weekend (and yesterday) in a final push of moving and cleaning, so it's a good day to start up our latest book recommendation conversation while I take a moment to catch up.

These days we've given up on those cutthroat polls (where the most recent movie adaptation with the slickest CGI usually ends up winning anyway) and instead we're just going to have a good chat about good books and all come away with some suggestions for our To Be Read™ pile. We'll still have the system of seconds (and "thirds" and "fourths"), but all that will determine is which goes to the top of the list when I post the results. You can go HERE to see what the results will look like when all is said and done. And I'll link out the original nomination post for folks who want to go see what people are actually saying about the book. Eventually these posts listing the results will be compiled in a massive "book recommendation" post.

Today we're doing stand alone classic science fiction. Those books that are not part of a trilogy, a series, or massive world building universe, but nonetheless are outstanding examples of the sci-fi genre. And since this would turn into a massive list if we did all science fiction ever, we're going to break it up into modern and classic. Today is classic—written before 1980.

The Rules

1. Make two recommendations. Obviously, I can't stop anyone from making fifteen, but nothing beyond the first two will make it onto the master list.
2. NO BOOK WITH A COPYRIGHT AFTER 1980 WILL BE ACCEPTED. We've do those in another conversation.
3. TELL US ALL A LITTLE ABOUT WHY YOU LIKE THE BOOK (or short story) although obviously do so without spoilers! If you just drop a title name and it gets all the seconds, I'm still going to list it, of course, but the whole point of this is to have a "conversation" and gush a little about the books you think are great, exciting, well written, or unforgettable. 
4. For each recommendation, let us know if you're nominating it more as a BEST book in the genre or an UNDERSUNG HERO in the genre. Basically "undersung hero" is for books you think are great, tragically overlooked, NEED to be read by everyone (like…yesterday), but are maybe not necessarily the besty bestest best. They'll all end up in the list I compile, but I'll put them in different places.
5. As always, I leave the niggling over the definition of genres to your best judgement because I'd rather be inclusive. If you want to nominate Dragonflight as sci-fi (even though it's probably better placed as fantasy), you should show your work if you desire those sweet, sweet seconds (or thirds....or fourths) and there might be a discussion thread after your comment with a lot of people writing out the "Ahem……If I may…"
6. Your book must NOT be part of a series or more than tangentially related to a fictional universe. If it makes a reference to another book like once or twice is clearly taking place in the world of another book without being a sequel, prequel, or a grand unified series, that's fine, but if it takes place in Discworld, that's not "stand alone." (Nothing wrong with those kinds of massive universes, but let's get them their own conversation.) 
7. You get to mention two (2) books. That's it. Two. You can do one BEST and one UNDERSUNG HERO. Or you can do two BESTS. Or you can do two UNDERSUNG HEROES. But two is the total. If you nominate three or more, I will, with unimaginable cruelty, simply ignore the third and any subsequent books. I'm sorry that I'm a stickler on this, but it's just lil ol' me compiling this list by myself and it's a pain when people drop a spinosaurus list of every single book they can remember in the entire genre that they vaguely liked at all. However, you list more than two books and your third or later choice gets a second, I'll consider everything. (Even though that matters a lot less than it did when I was counting seconds to see which titles made the poll––see below.)
8. Did I mention two?
9. You may (and absolutely should) give a second shout out to AS MANY nominations of others as you wish. There is no more poll, so this will not be a cutthroat competition to see who makes it to the semifinals. It will simply dictate which titles I list first, and it may influence which books someone considers a good recommendation. ("This one got six seconds, and that one only got two, so I think I'll start with this one.")
10. Put your nominations HERE. I will take nominations only as comments and only on this post. (No comments on FB posts or G+ will be considered nominations.) If you can't comment for some reason because of Blogger, send me an email (chris.brecheen@gmail.com) stating exactly that and what your nomination is, and I will personally put your comment up. I am not likely to see a comment on social media even if it says you were unable to leave a comment here. 
11. You are nominating WRITTEN fiction, not their A/V portrayals. If you thought Disney's The Sword in the Stone was a great movie, but never really could get through White's The Once and Future King, please nominate something else. (I love film, but it's a different medium.) 
12. Have a conversation, but check the typical internet assholery at the door. If someone likes something you think is terrible, it's okay to let them enjoy it. And if someone has one tight and polite bit of criticism about your recommendation ("I was not a fan of the X plot arc or the way that author writes women."), it's okay that they didn't care for it and there's no need to defend it like they have impugned you honor for seven generations.  I **WILL** delete shitty comments, and I absolutely know that's highly subjective, so better to err on the side of nice. 
13. TWO!