My recommendations would be:
David Brin - Startide Rising and The Uplift War. The first book in the trilogy “Sundiver” is good but not essential. And as another noted, the second trilogy is harder to get into.
Jack McDevitt - Really anything he has written. The*** Priscilla Hutchins*** novels are great, as are the Alex Benedict Novels. His one-offs are also good.
Larry Niven and Jerry Pournelle - most anything they wrote, either together or separately. If you’re not sure, start with The Mote In God’s Eye or Lucifer’s Hammer.
If you like space opera, Dave Weber’s Honor Harrington series is very good. As is Jack Campbell’s Lost Fleet series.
Gordon R. Dickson wrote some interesting stuff, including his Childe Cycle and The Way of the Pilgrim.
Spider Robinson’s Callahan’s Crosstime Saloon stories are a whole lot of fun.
Steven Brust tends more towards swords and sorcery type fantasy (but done a bit differently than most), but Cowboy Feng’s Spacetime Bar and Grill is more sci-fi-ish. I’m a big fan of his Vlad Taltos books. Also his Khaavren romances, while set in the same world as Vlad Taltos, are written in an Alexandre Dumas-like style, and are fun if you like over-blown dialogue.
If you’ve been living under a bush and didn’t see The Martian, definitely read the book by Andy Weir.
The Sparrow by Mary Doria Russell. A gripping take on man’s first contact with aliens.
Steve Perry’s Matador series is excellent, especially if you are a fan of the Martial Arts.
There’s a lot more, but this list would keep you busy for quite a while.