XII's would probably be the best. From what I remember, there's not a lot of cities or continents being leveled. All the major events seem large-scale political, so the everyday joe in a city like Rabanastre is fairly well off. VIII's isn't so bad, either, for the same reason.

X's provides a pretty bleak existence, but goddamn if Spira isn't easy on the eyes, so I can't call it the worst.

Worst is any one of VI, VII or IX's worlds. VI for the obvious reason. IX for similar reasons. VII's is actually not so bad after Advent Children and the Compilation were entered into canon (and swiftly blocked off from my memory). But back in the day I always interpreted VII's ending to involve a complete disintegration of humanity on the Planet. That's pretty ty.