Easiest: Final Fantasy XII
As much as I loved FFXII, I cannot, for the life of me, stress how much this game was too easy. Breezed through the main campaign with little to no difficulties. Boss battles tend to go by quick especially since at the end of the game, the party is basically so overpowered. The only notable problems are Yiazmat and Omega XII.
Other Notable Mentions: Final Fantasy II
Hardest: Final Fantasy III
What Sunny said, is basically straight to the point. Ah yes, one more thing... do I have to state how much I hate the game's final dungeon?
Other Notable Mentions: The DS version of Final Fantasy IV did have an unfair difficulty in terms of boss battles, while regular encounters are pretty easy... until the final dungeon that is.