I have read Ender's Game at least 25 times since High School. Only book I've ever finished the last page and started again on the first page immediately.
I enjoyed Speaker almost as much - but it is a completely different book. Speaker is much more philosophical. If you go into it knowing this, I think it helps. If you expect it to be Ender's Game 2 - well, the dude just isn't going to kill off an entire intelligent species again, is he?
Children and Xeno are both decent reads, but continue being philosophical and are not nearly as good.
My friends and I refer to the Shadow Books as "Card's vacation home downpayments". There is some good stuff in them, but they progressively get less insightful and meaningful. And the action more predictable and lazy.
In Order of my liking:
Game Speaker Enders Shadow Ender in Exile Xeno Shadow of Hege Children Puppets Giant
Note - I have not read Shadows in Flight yet, but will eventually.
Card does have some other good stuff worth checking out though.