I would probably pick the three D&D core books just because of the wide player base. As much as I enjoy other games, the support for D&D and the wide spread of it makes it excellent for getting to know groups and find games.
Eberron Campaign Setting
Sharn: City of Towers
Five Nations
Explorer's Handbook
(The last one is freaking fantastic! So many great ideas, and Rich Burlew helped write it, so it actually reads well, too!)
Ditto... I might drop the Five Nations (wasn't too impressed with it) and add the Expanded Psionics handbook.. as I have a new love for Psionic characters lately.