Expanse: I grabbed this on Kindle and loved it so I bought it again and have read almost the entire series. As I've been reading it again and watching the show on Prime, I've actually enjoyed characters in both versions. Particularly Ashford in the Prime show. I appreciate him much more than the Ashford we are given in the book. Whichever version you start with, you won't be disappointed by the stories told.
Gunslinger: This classic Stephen King series is about gunslinger who was modeled after Clint Eastwood. I've listened to the series on Audible more times than I can count and have been collecting original hardcover versions of the series as I can get my hands on them for a reasonable price.
Stormlight Archive: This is an excellent series from Brandon Sanderson. It follows a number of characters and spirits as they do battle. Without giving too much away, when you figure out who the destroyers are it's a great loop thrown into the story. I love how Kal works on his brokenness and failure. It feels heartbreaking and uplifting in turns.
Wheel of Time: This series was started by Robert Jordan but he didn't live long enough to finish it. He did realize he wouldn't get to finish it so left copious notes about the books that needed to get written with Brandon Sanderson and Jordan's wife. With Sanderson writing and Robert Jordan's wife editing just like she had for her late husband the series ends in a spectacular way. The only caveat is that books 4 and 5 are slow, but there is so much good story after that to make the slog worth it.
The Silo Saga: While I don't read much horror, this is technically in that genre in most stores. What if the world needed a reset. What if a billionaire had a plan and co-opted people into it in various silos? What would happen to them if order broke down? Read the story to find out.
Ender's Game: Ender's Game is the most recognized book, but really the first 4 books are excellent. I own Children of the Mind in hardcover, and the rest on Kindle and in Audible. I've read them all multiple times and it's one of the few science fiction series my wife very much enjoys.