Easiest: VII
Hardest: IV
Hardest : Final Fantasy XII
Easiest : Final Fantasy VII
Why?
Final Fantasy XII has some really hard boss fights. Not to mention it has a low amount of gil to be made as well. The chance you will miss out on something is very high. You don't have to do all the side quests, gear finding etc but if you do you'll notice in other Final Fantasy games getting the item you want is as easy as knowing where it is. There is chests that have 1/1000 chances of holding the item you want. The bosses can be challenging and level grinding is a pain at times. Overall I spent more time on this game then any other Final Fantasy.
Final Fantasy VII has a very user friendly system that allows you to become overpowered. Also there are moments where it may seem difficult but with a little leveling the bosses can become easy. It seems very commercial and allowed a wide range of players see what Final Fantasy is capable of. I don't know many people who get lost in this game and all the items is pretty easy to obtain.
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Final Fantasy II is hard cause of all the time it takes to actually level up your members. Not to mention you get one of your members real late in the game. The limit to how many items you can carry restricts you on which ones you keep. I mainly hate this game cause the leveling system is too difficult for me to abuse.
Final Fantasy XIII is both hard and easy. While most of the story line doesn't take much leveling up, the optional stuff can take 100 hours. You can pretty much get through the games with COM/RAV/RAV. Still leveling up all your weapons and accessories is the big thing to do. It lacks ways to make obscure amounts of money but has ways to make some more then 12.
Final Fantasy X now and days pretty much is the first installment that required 100 hours of gameplay. The game can be beat when each player finishes their portion of the sphere grid while you can also grind and learn the whole grid for everyone. After that the superbosses can be a pain but for completion sakes you don't have much trouble in Final Fantasy X.
Marcus
"No cloud, no squall shall hinder us!" - FF9
Games completed - 1,2,3,4,5,6,7,8,9,10, 10-2, 12, 13, 13-2, 13-3 Tactics, Tactics Advanced, Crisis Core, Dirge of Cerberus, KH, KH2, And played 11.
I felt like FFXIII held my hand throughout most of the game, you almost never lose if you have a decent sentinel character during boss fights and it had that auto battle 1-click thing where you didn't even need to choose your moves...I don't ever remember struggling with XIII.
Easiest: X-2
Hardest: X-2 Last Mission
I like Kung-Fu.
Out of the ones I've played I think the easiest is IX with the tower defense game being the hardest. III was a struggle, but I did beat it in the end. Never beat that RTS game.