Please, not the Realms. The feel of the Realms is very different from that of the new D&D, and the setting has suffered more than enough changes already to accommodate it to the roolz.
And while Eberron has a much stronger 3E synergy, it's too much of an acquired taste to provide baseline D&D imagery, and deserves the chance to flourish in its own right.
While I'm not fond of the World of Greyhawk's treatment in the 3E books, Greyhawk Light, the variant World of Greyhawk that's the current 'default setting', is probably the best choice.
And while Eberron has a much stronger 3E synergy, it's too much of an acquired taste to provide baseline D&D imagery, and deserves the chance to flourish in its own right.
While I'm not fond of the World of Greyhawk's treatment in the 3E books, Greyhawk Light, the variant World of Greyhawk that's the current 'default setting', is probably the best choice.