I’d agree with you there. I read those in middle school and would suggest them to anyone, just starting to read sf or not.
A few others, mostly aimed at precocious younger readers. Because that’s what I was (helpful, eh?):
- Green Hills of Earth, Heinlein’s '51 short story collection. A good example of classic space-and-atomic-power sf.
- Tales of Pirx the Pilot by Stanislaw Lem (translated from Polish). Humorous in a slightly less… silly? way than Harry Harrison’s Stainless Steel Rat or Bill, the Galactic Hero series. Lots of interaction between man and machine, but in a more human way than a lot of hard sf.
- Devil on my Back by Monica Hughes isn’t the best book in the world, but as a kid it was my favorite of the brain control/societal control sorts of works. More young adult than the rest of this list.
- Hitchhiker’s, of course, I agree with that.
- Martian Chronicles, because everyone should be exposed to the way Bradbury uses language and creates worlds.
Okay, so that’s a few off the top of my head.