I had to vote Greyhawk.
Because it's a non-entity. Eberron, while flashy, has alot of ,not just non-core, but non-genre material in it. It's very SPECIFIC. It is, at its foundations, a NICH PRODUCT for an extremely nich game.
To make Eberron "D&D", I.E. when you pick up D&D you pick up Eberron, would be alienating way too much of the potential market. Not that Eberron doesn't sell, mind you, but I think it needs D&D RAW to kick start interest in it. Greyhawk WORKS because you can have all those bazillion little free adventures from WotC that really have nothing beyond the immediate story-line to place them ... and you can place them. Same with modules, same with your own adventures. That becomes harder to do when you have very culture-specific rules in place ... like Dragonmarks.
In essence, I think the simpler the better.
Average man-off-the-street doesn't know what a Dwarf is, maybe. But average Dork-Off-The-Street probably does. The guy who played WarcraftIII and World of Warcraft and is now interested in RPGs, he can "get" Greyhawk ... but the average person DOESN'T know what a "Warforged" is or why sentient "constructs" are odd or what a "Dragonmark" is or why that's different or why that's the same as Vanilla Fantasy. Action Points are beside the point ... if they make a 4e, maybe that's a good enough idea to catch Core RAW, but I don't think some core things like: "Well 'constructs' are built machine-like things created with magic, based originally off of golems, which is an old Jewish myth, but which D&D expanded to take in Frankenstein, and which has been further expanded into a minor sub-theme of Magical Technology, but a defining aspect of these "constructs" in the game has been that they have no Constitution score and are fully under the control of their creators while Warforged are constructs as well but they DO have Constitution scores and are free-willed" ... that whole long-winded concept sort of ruins the "Core Setting" appeal. We all "get" Warforged because we already "get" core D&D.
--fje