Here's my list:
1. Lords of Madness - Great ideas, mostly flavor text, societal stuff, and important information about Aberrations. Won't be much use if you're a player, but as a DM... it's fantastic.
2. Complete Arcane - Personally, it's my favorite of the splats. It inspires me quite a bit more than any of the others and it has a lot of cool ideas. Then again, since one of your players already has it, you may want to wait.
3. Races of Eberron - I don't actually have this yet and I haven't really read through it... at all, but it looks good and you don't need the Eberron CS book to use it!
4. Sandstorm - Get this ONLY if you want the desert hazard/weather rules. The rest of the book is rather boring and uninspiring... except for the Dry Lich template. But that's it really.