Ender's Game is my favorite adult sci-fi book series but I can't actually recommend it to anyone because the author is a terrible person and I don't want him to get any money. If you're able to borrow it from a friend or the library check it out! But please don't buy it, seriously, the guy is a total jerk.
None of my irl friends read sci-fi so I'll pass on some recommendations here.
Leviathan Wakes by James S. A. Corey - Space opera, huge books but read quick, like a movie, very good tv series version with 4th season about to come out on Amazon Prime
Blackfish City by Sam Miller - Kinda SF kinda fantasy, post-apocalyptic near future environmental dystopia, one of the best books I read in 2018
The Long Way to a Small Angry Planet by Becky Chambers - Interplanetary war as a backdrop but tightly focused on the relationships between the crew of a small space ship, very domestic, not action-packed but very emotional, antidote to overly technical SF
Ninefox Gambit by Yoon Ha Lee - I love this book, it blew my mind but I won't lie it's a hard sell, if you like hard SF, confusing math, explorations of gender as a concept, and more ethical dilemmas than you can shake a stick at, this is one of the best SF series I have ever read