So much to say...
I don't consider myself a sci-fi aficionado, but I really like good fiction done well. That means good imagination coupled with good writing. When I was a kid, I read all of Frank Herbert, and that was enough to put the sci-fi hook in me.
That said, if you haven't read Herbert (Dune Trilogy), do. He was a real master and influencer. The books are still riveting, imaginative, masterful.
https://en.wikipedia.org/wiki/Dune_(novel)
Beyond Dune (joke right there), all his novels are generally good and thought-provoking.
You said you read Heinlein, but have you read "Starship Troopers" in the last decade? You should. I did a few years ago and it took my breath away after 20 years. It's a way more sophisticated, politically and culturally astute book than it appears when you read it young. The terrible, comic-book joke movie in the late '90s by Verhoeven ruined the franchise; he had no idea what the book was really about and made a mockery of it.
I loved Douglas Adams when I was young, but it's more "fun, fluff" and not serious fiction. That said, it's a genius piece of work and changed comic writing after it. I feel actually privileged to be OLD enough to have read it when it came out and seen how it changed comic writing. But I don't think of it as "real" sci fi.
I do like the expanse series, but it's so derivative of George R.R. Martin it eventually tires me. It was so much like martin (great ideas, very badly written) I looked up the authors and found out one was actualy GRRM's executive assistant - he just copied GOT into sci-fi and copied his bosses style. I do really, really like some of the ideas put forth, but the writing is terrible and so derivative. It is a triumph of connections and marketing over content. But that said, it's entertaining enough. I even watched the TV/cable show, but there IS better stuff out there. I have read all that is published to date but I will not bother with more; there is better stuff out there.
A lot of people love Orson Scott Card - I am on the fence. There is no doubt "Ender's Game" is a legend, and it is worth the read.
https://en.wikipedia.org/wiki/Ender's_Game
The follow-on novels become more preachy and more difficult. His "Homecoming" series ("Ships of Earth", etc.) are interesting, but some find the over-arching theme of his Mormonism overbearing. I won't judge, but start with "Ender's Game" and see what you think. But do read it if you have not.
Which brings us to the greatest Sci Fi writer in English I think - Iain M. Banks.
https://en.wikipedia.org/wiki/Iain_Banks
This guy, who very sadly died of cancer relatively young a few years ago, was a literary and intellectual giant. Titles of sci-fi genius go Heinlein-Herbert-Banks-Liu (more on that below). He was a dedicated SF writer, but got no where. In the 80s, he switched to mainstream fiction and totaly rocked British fiction with "the Wasp Factory" and "Espedair Street." (this is how I found him). Then when he had all of English literary sets adoring him, he switched back to SF and blew their minds. His "Culture" series is worth reading every book. I will say no more as I could go on for ever.
Before I get to the last I have to acknowledge some things said above. Neal Stephenson had some GREAT early work. Snowcrash is worth reading. Cryptonomicon is a great WWII alternate history (it is not SF). Cryptonimicon starts to prefigure his obsession with conspiracy and his own superlative genious, which of course shows the past-peak nature. Diamond Age is a tiresome, windy piece of sound and fury, save your time. Other than that, walk away. But do read Snowcrash, and walk towards Gibson (who he was consciously copying).
Gibson's Cyberpunk trilogy will go down in history as massively important, and it is GOOD. Neuromancer, Count Zero, Mona Lisa Overdrive - incredible stuff. Read it and all modern SF - TV, etc., all seem just so derivative (as is Stephenson, who just copied it but at least hit a nerve with Snowcrash). Hugo award stuff, Gibson. I count myself lucky I read it in the early 90s and saw what it did. I got to meet Gibson once, c. 2002 late fall, he gave a talk at Elliot Bay Book Co. in Seattle. I lived a block away at the time and went out to walk my dog and happened to stumble on a sandwich board announcing his talk and went into the basement cafe for it. What a shock and gift - like waking up one day at age 40 and finding Santa had come in the night and given you tons of lovely fun gifts. I mean, what a surprise and joy!
Another name I think will become legendary is going to come out of the blue I think - Cixin Liu.
https://en.wikipedia.org/wiki/Liu_Cixin
The guy won the Hugo award, so it's not like he's unknown. But he is Chinese, and his works are only available in translation. He is undoubtedly "hard" sci-fi - it's technical, it's fantastical, and it's awesome. He is basically the Heinlein of China, but living now and writing. And he's good. His most famous book "The Three-Body Problem" and the following two books are just awesome. You have to spend about 1/3 of the first book getting used to the translation and it's limits, but once you get hooked on his ideas and very real-feeling embrace of the "first contact" conundrum, you are hooked. He's a genious and worth reading, even if in translation. In some ways, he is very reminiscent of Weir (Martian), as science is never "hand waved" away and is front and center. If you liked the Martian, you will embrace this even though it goes much father and is in translation.
OK, I am skipping a lot of good people (Dan Simmons, others), but those above would be my very top recommendations to keep someone occupied a long time. I read a lot. I am not really an SF afficianado, it is less than 10% of my total reading, but the books and writers above really do stand out.
