Children of Time

There’s lots of answers to this depending on taste, but you also get into arguments about whether such and such is space opera or planetary romance. Children of Time is hard SF the way a reader from the 1960s would have understood it.

Peter F Hamilton has some great hard sci-fi novels like the Commonwealth series. Super nerdy with some interesting characters.

John Scalzi is probably my favorite sci-fi author for excellent characters. His “Old Man’s War” is genius.

I liked Daemon by Daniel Suarez, I read it many years ago but it’s more relevant now than ever (the story is about a rogue AI).

Iain M Banks Culture series

The Mote God's Eye

Anything by Asimov

Also there's a lot of great short stories in this genre. For example the road not taken by Harry Turtledove

Dune, Children of Time, Neuromancer and Blindsight.

for "sci-fi" that reads like fantasy, the Sun Eater series is really fun.

Project Hail Mary is light reading sci-fi. Which is fine, I enjoyed it, but if you are looking for something meatier, covering broader themes and character development, here's some other recent stuff (there's a lot of old stuff covered already in other comments):

Stories of Your Life and Others; Exhalation (Ted Chiang) - both are short story collections vs novels, though

Dissolution (Nicholas Binge)

Too Like the Lightning (Ada Palmer) and sequels (wordy, philosophical, interesting future society)

Tell Me an Ending (Jo Harkin) - more near-future and grounded

Void Star (Zachary Mason)

If you want interesting worldbuilding concepts of near-future international politics, Ray Nayler is your bet.

If you want "sci-fi your dad would like", Scalzi is your bet.

If you want hilarious, but heartwarming deconstructions of common scifi tropes and protagonists, Martha Wells' Murderbot is your bet.

If you want a comforting read, you'll want Becky Chambers.

If you want a wild romp of science fantasy, you want Tamsyn Muir.

If you want math-as-magic-scifi space opera, you want Yoon Ha Lee.

And of course the most wildass mililitary scifi, Kameron Hurley is the queen.

I have personally been going through and enjoying Alex Gonzalez's "> rekt", which is a novel about chilling brainrot.

So, I should more ask you, what is your definition of "great"?

If you define quality as "layered and meaty" there are many much better books.

Roadside picnic (and its less Russian counterpart, Annihilation), left hand of darkness, Solaris are all excellent.

If you want culturally influential, surely Starship Troopers, Stranger in a Strange land, anything by HG Wells, 1984/Brave New World, Frankenstein (duh)

The characterization in Hail Mary is just so damn weak, even space opera stuff like Bujold

I would add Vernor Vinge's A Fire Upon the Deep and A Deepness in the Sky to these suggestions

Dragon's Egg by Robert Forward

It's my point-to book for friends asking about science fiction as a genre.

Depends a lot what you are after, but look for writers like Dan Simmons, Arthur C Clark or Alastair Reynolds.