The Martian is certainly one of the best pieces of science fiction of the 21st century and probably the one most likely to still be read in 50 or 100 years. (Definitely not you, Ready Player One.)I have to agree. Not only did this thread devolve too early, it was really low quality navel gazing, at that. Oh well, it happens.
On topic, and to your first point, I will offer "The Martian". Maybe not the #1 sci fi novel of all time. But probably #1 in the past 20 years. Also, to the main point if the thread, it's a stand alone novel as opposed to a series.
I'm also a little surprised we've gone this far without mentioning A Wrinkle In Time (unless I missed it).