I voted for the Realms, because that's the setting that has always been D&D to me. I came into the hobby in 92, and after a formative year in Mystara, I discovered FR. Fell in love with it straight off the bat, and it's been my favourite ever since.
Also, I think it helps to have a default setting that will actually see some support from the publisher, but doesn't come with as many tweaks to the base game as Eberron (although I do like Eberron).