I'd have to go with Summerset. I'm still part of the crowd that thinks Skyrim has quite a bit of potential for interesting locations (certainly, of the more "Earthlike" provinces, it can be the most interesting alongside Hammerfell), but part of me wants Bethesda to tackle another exotic province ASAP immediately after their work on Oblivion just to help get their creative juices flowing again...and to put us all at ease for a while over doubts on just how "creative" their worlds have been as of late.
EDIT: I think Hammerfell could be quite interesting as well. It probably would possess the second largest Dwemer contingent after Vvardenfell, and there's always the possibility of throwing in some Orcish culture as well (assuming Orsinium expands south, past Wayrest). Maybe even some Sload? Regardless, I think we could get some nice geographical variety out of deserts, rocky highlands, savannah, jungle, et cetera.
Summerset Isle might be temperate and paradoxically have more of an Imperial presence which means nice buildings and other nice motifs of Imperial culture.
Heh, yeah - in many ways it feels as though the Imperials had stronger cultural identity in Morrowind than in their own native province during Oblivion, but that's probably just due to the heavy contrasts they had with the conservative and traditionalist Dunmer areas.