Tough call. I can only say that each story arc must be read in order, that being the Shadow arc and the Ender arc.
ENDER'S GAME is ground zero, and should always be the first book of any to be read. From there, I don't think it really matters if you choose to read ENDER'S SHADOW and the following books next, or SPEAKER FOR THE DEAD and the following books next, as long as you read each of their sequels in order.
Personally, I read them along the lines of publishing dates, and enjoyed them all immensely that way.
The actual chronology goes something linke this:
1. ENDER'S GAME/ENDER'S SHADOW (overlap)
2. SHADOW OF THE HEGEMON
3. SHADOW PUPPETS
4. SHADOW OF THE GIANT
5. ENDER IN EXILE (forthcoming)
6. SPEAKER FOR THE DEAD
8. CHILDREN OF THE MIND
Wow, thats great. Thanks for the concise reply. I think I'll go with Ender's Shadow next.