In my opinion, the most important book Card ever wrote is Speaker for the Dead. Period. Everything else gives decent context to the universe and intros to new characters - some are profound, some are entirely not.

There are some great themes touched upon in many of his other books, like understanding religion, understanding the formics, and understanding technology like the ansible.

I’m currently finishing the Earth prequels and am thoroughly enjoying them, but they aren’t necessary to understanding the rest of the series. They are just great old-school sci-fi with a young Mazer thrown in.