Easiest: VIII, without a shadow of a doubt. I see everyone talking about how "if you don't level, it's insanely easy" and I did level, I fought loads of random enemies, and it was stupidly easy. You just use the system it gives you. You can Card Mod yourself 100 Hero Potions and use them to be invincible for the entire final dungeon if you want. The good thing about VIII is that despite all this, it's still fun and has interactivity with it's limit break system.

Regarding XII, I feel it was easy a lot of the time but it was a massive grind, too, for a single player game. I feel this gives it the feel of being a difficult game because you don't want to grind and this leads to you facing bosses that you are not leveled appropriately for. At least, that's how it was for me.

Regarding XIII, you might have to choose between various paradigms but it's not simply Tank Attack Heal, that's for certain. It's Buff Debuff Stagger Attack Heal Tank. If you think you can play the entire thing without utilising both Stagger and Attack then power to you, but you're doing it wrong. Compare to that to every other game in the single player FF series where it is not Tank Attack Heal - it's Attack Attack Attack Attack Attack Attack (Maybe Heal) Attack Attack Attack Attack Attack (Maybe Heal). I honestly think 95% of my fights in the single player FF series outside of XIII have been fought by holding down one button. Seriously. XIII may give you a "smart battle" button, but it still requires far more input than the previous FF's I've played (which is basically every main FF) outside of the online games.

Hardest? XI and XIV, without a shadow of a doubt.