If I had to rank the ones I've played it'd probably go as follows:
Hardest
FF4
FF5
FF1
FF2
FF7
FF6
FF9
FF8
FFX-2
FFX
Easiest

I can't put FF3j on there, as I have not played it, and I don't feel that FFXI, FFT, FFTA or FF:CC fit in there as they're sort of different.