The Handmaid's Tale by Margaret Atwood
The Handmaid's Tale is an absolute classic dystopia - first published in 1985, it still feels chilling and prescient today. Although it's set in the future and definitely a dystopia, it's usually counted as "literary fiction" rather than sci-fi / fantasy. There's an unfortunate snobbery at work, where some circles refuse to consider that any sci-fi or fantasy could be literary, and if something is clearly literary as well, then it somehow gets elevated to "not sci-fi / fantasy" even though it clearly is. The same happens with Atwood's other books (in fact even she says they're not SFF) and with Kazuo Ishiguro's Never Let me Go. It's a shame that people insist on this snobbery and these silos - but if anything, it goes to show how very vast the genres of "imaginary worlds" actually are.
What this book does especially well: Atwood's prose is beautiful, pure and resonant as a crystal chiming (hence the literary accolades, which she certainly deserves) and its theme is strikingly powerful.
A Game of Thrones by George RR Martin
A Game of Thrones barely needs any introduction any more, though you may not know that that's the name of the first book only, and the full book series is called A Song of Ice and Fire. And if you're feeling impatient for the next instalment of the HBO series, spare a thought for readers who started the books when the first one came out in 1996!
If you only know the story from the TV series, or were put off the TV series by the excess breastage, then I highly recommend the books. The TV series has done a fantastic job of condensing and adapting the story, and the casting is particularly rich, but there's a wealth more detail in the books, and they are absolutely not sexist at all. Everyone suffers, and the forces of war and patriarchy play out exactly as they genuinely would, but this is in no way misogynistically done, and if anything it's an extensive critique of our feudal-fantasy dreams, where we fill fantasy books with castles and wars, not thinking carefully about how that would actually work. These books take it seriously.
Also, while the TV series is all about the sex, the books are much more obsessed with food. Seriously: lay in plenty of food before you start reading. In particular, you'll need roast chickens, wheels of cheese, vegetables drowned in butter, stuffed chillies, and loaves of fresh bread. For starters.
What this book does especially well: George RR Martin's world is exceptionally well realised, with vast detail in every aspect of culture and people's lives, and close attention to the ripple-out effects of every choice.
The Three-Body Problem by Cixin Liu
This is hardcore hard sci-fi, taking particular scientific principles and exploring them fully. If you're happy reading A Brief History of Time and the like, you'll love this - it's chokka with genuine science, fascinating extrapolations, and physics dilemmas. But if physics isn't completely your thing, you might find this hard going, because lengthy passages are devoted solely to the physics, and the story wouldn't make sense if you skimmed them - the science is the story. A Marmite book, then! Incidentally, it was translated from Chinese by Ken Liu, author of The Grace of Kings, who's also on the recommended list.
What this book does especially well: it uses meticulous science to develop an idea that would otherwise seem outlandish and even cliched, it embeds the political realities in its characters' lives, in the future it imagines as well as in the historical past, and its theme - whether or not humans even should be saved - feels painfully relevant at the moment.
Ash by Malinda Lo
Ash is a children's book (shelved in the 9-12 range) retelling the story of Cinderella, with a few twists - most significantly, in dismissing the traditional trajectory. If you've read Angela Carter's fairytale retellings in The Bloody Chamber, then the "twists" in Ash might seem tame in comparison, but remember who the book's readers are: as an intro to feminist fairytale retelling, this is captivating, and reminds you that you're always allowed to rewrite the stories yourself. Its world-building is light, in keeping with it being fairytale rather than epic fantasy, but the description is still vivid and its use of magic is deft, interesting, and well-described.
What this book does especially well: overturning the usual familiar tropes of its genre and the obvious choices for an unexpected approach.