Argyle King
Legend
Fluffwise, I like Dark Sun and the grittier tone. However, I think the Realms match up best with the mechanics of 4E and the ideals behind how the game is built.
Of these, my favorite is probably PoL. It's vague enough that I can toss things in all willy-nilly, but still packed with thematic conflict.
I like the PoL cosmology and mythic history a lot - it's got great flavour and conflict built in (my contrast would be Greyhawk - a lot of flavour, but not so much conflict).It's funny: I like the mythology behind the PoL setting a lot more than I like the setting itself. The various settlements and lost nations of Nentir Vale don't really grab me - to a large extent, it doesn't feel "real" enough to me - but the mythology is great. Primordials? Gods? Fantastic!