III was really the only one that was tough, because it doesn't have a quick leveling up trick. Every other game does, and because of that, they lose difficulty. Especially FFII XD (That's why I laugh when people think that FFII is difficult )

Even then, FFIII is only really hard if you're unprepared, then it would look very difficult.

And of course, all the FFs VI on up to X are easy. Probably why they added in optional stuff in the later ones.

So I'll vote FFIII. It was more "normal" to me than hard, but It could get difficult at times.