Answer:

Preferred order ... depends on what you mean by preferred, and who's doing the preferring.

You can read them in the order written - Ender's Game, Speaker for the Dead, Xenocide, Children of the Mind, then Ender's Shadow, Shadow of the Hegemon, Shadow Puppets, Shadow of the Giant. Or you can read them in chronological order of story beginnings, which is Ender's Shadow, Ender's Game, Shadow of the Hegemon, Shadow Puppets, Shadow of the Giant, Speaker for the Dead, Xenocide, Children of the Mind.

But in truth it doesn't matter, except that you should read Xenocide right before Children of the Mind, since they are really two halves of a single continuous story. In most of my books, I include all the information you need.