Virtually every book in this thread that I've read is a good book and worth reading, but I'd personally separate that from "must read". For me, must read means two things:
1) Books that will be remembered for decades, or that will be or already have been significantly influential on SF/fantasy writing.
2) Books that have just been absolutely huge and regardless of quality are influential on audiences and general perceptions of SF/fantasy.
That means very few of the books mentioned so far would qualify, and not necessarily the ones I like best!
For genuine must-read, which again, not necessarily my top faves, I'd say, in no order:
- The Broken Earth trilogy by NK Jemisin - basically an "instant classic". It was literally like reading a classic SF series, it's so self-assured, so confident, so well-executed. I don't think her two books since have been as amazing, but that's always the way with SF authors.
- The Kingkiller Chronicles by Patrick Rothfuss - Not a huge fan personally, and I doubt there will ever be a third book, but this definitely spoke a lot of people, including outside of main core of fantasy readers. Largely because, I'd suggest, it's basically fantasy + Charles Dickens. But certainly the first book is important to read I think to understand why people are obsessed with it.
- Mistborn OR the Stormlight Archive by Brandon Sanderson - I think Sanderson is a sort of a mediocre writer, but that's kind of never been a big problem for SF/fantasy audiences. Certainly his work is going to be quite influential over the next couple of decades, I'd suggest. Either work showcases his highly developed magic systems, good fight writing, and both also show his various flaws as a writer (which the audience largely ignore).
- The Expanse series - Again, not super-keen on these, don't rate either writer as writers, and it's barely SF a lot of the time, because it's so focused on "political thriller" elements - but again this probably helps it with the broader audience, because it's comprehensible to people who don't really "get" the SF elements. Likely to be highly influential. Certainly don't need to read all of them.
- The Southern Reach trilogy by Jeff Vandermeer - The first and last are both brilliantly written and likely to both influence future SF writers, and open the minds of people who read them.
- The Hunger Games trilogy by Suzanne Collins - Good pick mentioned by @Zaukrie which I'd otherwise have missed. Essentially the definitive YA series, which countless others since follow the template of. Unspeakable levels of influence, and not bad books in their own right.
- The Imperial Radch novels by Ann Leckie - Some fantastic writing and also truly 21st century in their approach.
I'll try to add some more later.
There are a lot which I'd like to add but feel are "unproven" as yet as to whether they will in the longer-term be "must reads" for the 21st century (particularly the Locked Tomb series). There are also quite a lot of books form the around '00 and the early '00s which feel profoundly "last century" - American Gods is the only one I'd call out right now - it's such a 20th-century book. It fits far better into the SF/fantasy of the mid-1990s than the '00s, and that's not a good thing. I read it not long after it came out and it felt like I was reading something distinctly dated. Part of that is of course because of TT RPGs, which explored virtually all the same concepts, and better, in many cases, than American Gods.