X was the easiest IMO. Once you get deep into the Sphere Grid, you can really beat the game with three characters.

I wouldn't consider VIII easy just for the fact that some stuff in the game(i.e. Boost, getting Doomtrain,) I didn't even know about until I read a guide for it. But then again, I did handle the final boss with some ease.

VII was also easy to me, seeing as I beat the game without Vincent, without any ultimate weapons, and without any ultimate Limits. Hell, I didn't even have the Knights of the Round summon or half of the summons in the game for that matter.