XIII was one of the harder ones for me, especially once I hit Pulse. I died more times in that game than any other game.

I said VII is the easiest because I never refine magic to make my characters in VIII basically unbeatable, I only use magic I draw from enemies, which balances the game more. IX wasn't super difficult, but it took some grinding in certain spots. VII I never had to grind and they just hand you levels. It's a serious task for me to get to lv. 99 in IX, and in VII I barely had to make an effort. The characters level up so quickly and their HP just goes through the roof pretty early on. I didn't fight the WEAPONS, which could have made it harder, but the rest of the game was just a cake walk. Which is good because I didn't want to spend any more time than necessary playing it.

There were a few spots that gave me a bit off difficulty in VI, XII actually gave me a lot once I hit the Phon(?) Coast. I've heard IV on the DS is easy, but I haven't beaten it yet, so I can't say for sure.