It seems to me as if Planescape is the natural core and default setting. For one thing, Planescape leads to all other settings, so if you really wanted One Setting to Rule them All, Planescape would be it.
With all these other default setting worlds, you kind of have to do some maneuvering to shoehorn in all the core races and classes. Ie. if you want eladrin to play a part in the Forgotten Realms, you need to come up with a story about how the Eladrin suddenly became prominent. Planescape is the Mos Eisley of D&D, and you can literally find every race and class in it.
With all these other default setting worlds, you kind of have to do some maneuvering to shoehorn in all the core races and classes. Ie. if you want eladrin to play a part in the Forgotten Realms, you need to come up with a story about how the Eladrin suddenly became prominent. Planescape is the Mos Eisley of D&D, and you can literally find every race and class in it.