Depends on what you're into. Ender's game is obviously the first book in the whole series and kind of sets you up in the universe. But if you follow Ender's path and go down that line of stories it's a lot of heavy dialogue and interplanetary political stuff

The Shadow Series follows Bean, which is another character in the book, but it stays on Earth and of deals with the aftermath of Ender's Game. Still a lot of dialogue in politics, but it's interspersed between a little bit of action.

To get the whole story, you need both timelines. But I tend to like it beans timeline a bit more.