Xenocide

I'm surprised to say this is the strongest so far in the Ender's series. It's a little more philosophical than the others but in a way that suits what's happening on the page plus it also includes a fantastically realistic anti-heroes journey I didn't expect.

I'm really not sure about the whole young Peter and Val surprise, it felt like it came out of nowhere purely for the purpose of sequels

Considering who wrote this, it's fascinating to see the issues of religious fundamentalism discussed in such detail.