I go completely for the Telvanni stronghold, AKA Tel Uvirith, namely because I think it looks cool, plus there's more mods for it than any other stronghold(Uvirith expanded, Uvirith unleashed, Uvirith vault, Uvirith underground, Uvirith Inside, etc.), or at least to my knowledge anyway
As for climate and all of that, that's not really a problem, as anyone with internet (which obviously includes anyone making or thinking about the descision) can download Green Uvirith and its patches and updates, which turn the surroundings of Tel Uvirith into something just as nice as what the Ascadian Isles can offer (turns the ash into grass, adds plants, adds trees, adds a imperial wall around the area, adds a little town-thingy, adds a FRICKIN' DWEMER OBSERVATORY, and a silt strider too)