NOTE that I did not ask "what are the BEST science fiction books". I am talking more about accessible and enjoyable as an introduction. For example, recommending The Three Body Problem (or if we want to stretch the definition of SciFi a bit and include something like Gravity's Rainbow) to someone who's never read science fiction before, you'd almost certainly be turning them OFF to science fiction for good (opinion)
@codeWhisperer True about the Three Body Problem,
A Wrinkle in Time,
Any Terry Pratchett
(I was started out in 6th grade with The Martian Chronicles)
@codeWhisperer Lois McMaster bujold, first barrayar book. Simak, time is the simplest thing. Sturgeon, more than human.
I put up the list that I did because I think they are all accessible and a good mix of classic, modern, opera, etc. They are all engaging, yet none beats you over the head with worldbuilding.
I'm torn on OSC, so I didn't list it - yes, he's problematic, but Ender's Game is absolutely brilliant. Cosmic Jr. loved it and it's how I got him to read sci-fi.
@codeWhisperer Old Man’s War by John Scalzi