Going from best to worst.

Best-

Spira (FFX) - So if I die, I either get sent to a paradise world in the Farplane, reincarnate into a snazzy monster that can eat people, or transform into snazzy Jedi Zombie that can't be killed and is 10x more powerful than when I was alive? Seriously, why are people fearing death in this title again? When you kinda have a guarantee how the afterlife works, I can't imagine why death is considered a big deal anymore. Despite the rampant death, its trauma is negated by the afterlife system being more cool than terrible.

FFVIII - Except for the proxy wars and the Lunar Cry, VIII probably has the most peaceful world in the series. Even when cities are taken over it doesn't feel like a big deal like it does in other worlds.

FFV - Ignoring the global Crystal crisis and that one tiny moment when most of the world got swallowed by the Void, V has a pretty damn happy and friendly world. Even the places that got destroyed come back the same as before and except for having to relearn where everything is, its a pretty big and epic world.

Blue Planet (FFIV) - One Imperial power causing problems and another crystal conspiracy but only three locations get wiped off the map and at least two can be recovered. Half the cities get smacked around but nothing as bad as other titles.

FFIII - Most of the world is frozen in time and pretty much every city has some asshole servant of Xande stirring crap up but half of them are laughable while the other is actually pretty serious. Most of the monsters are just causing havoc and there is very little actual bloodshed to speak of except for a few locations. More annoying than terrible in my opinion.

FFXII - Wars, monsters that want to eat you and others that want to transform you into undying servants... It has a lot of potential for greatness but also has some pretty terrible scenarios as well. Though the afterlife is kinda known, it seems more like a bleak existence to me...

FFII - Evil empire that brutally conquers its territories, lets monster loose around the world and kidnaps and brainwashes war victims to become their soldiers. Not a very happy game until Emperor Mateus is gone.

FFVII - Another brutal regime that uses brutal methods to conquer and keep secrets. Half the world is already in bad shape thanks to Shin-Ra and even death isn't much of a comfort cause they use the souls of the dead to power people's electric toothbrushes. Meteor and the WEAPONS are pretty nasty diversions but I feel Shin-Ra has done more to make VII a bleak world. Even their destruction probably caused more harm than good.

FFIX - Wars, Mist powered monsters, Civilization is kinda small and holding onto a thread by the end thanks to ever major city being destroyed. Most of the planet is uninhabitable and you've got aliens killing you and then screwing you out of the cycle of life and your chances of reincarnation... IX's world sucks. There are still a few untouched and peaceful places but most of the world is in pretty bad shape when its all over and once the Ilfa Tree gets destroyed most of the world's Mist powered technology goes with it.

Worst -

FFVI - Oppressive Imperial empire with a psycho clown brutally destroying crap as well as performing unethical genetic experiments. Release of ultra powerful Espers and monsters that wreck havoc on the entire planet. Entire planet gets destroyed, monsters mutate into super monsters, mad man playing god and destroying towns, huge population lost and mass extinction. Even with Kefka gone and his will no longer hampering the planet from recovery, it doesn't change the fact that most of the planet got destroyed and turned into a bleak 80's post apocalyptic world. Its the War of the Magi on repeat.