The Ender Quintet is brilliant.

(Ender in Exile was written later and is an "extra story", falls between the first two.
One thing I've found interesting is how much Card predicted- so much of the technology used in the stories mirrors what we have now, and these are books written in the 80s. There are things which resemble computers and tablets, Skype/Zoom/Teams, VR games, using those games for training soldiers, information sharing and communications over what he calls "The Nets" ...
Also interesting, especially in more recent years, is how two bloggers managed to influence politics (put in a way to avoid spoilers).
There's a lot of philosophising throughout the books, discussions of ethics around various topics ... gah, they're just great.