Out of the main Final Fantasy series, I have completed nine of them(And I'm not that far off the end of FF3) and I can honestly say that Final Fantasy X is the second most easiest one I have played(A couple of bosses killed me, not often though), with FFVIII being the easiest.

So, why is FFX so easy? This is all because FFX was aimed at a new generation, and has introduced many to Final Fantasy. I'd love to see one of the people that started on X try playing FFI or II, but maybe that's cus I'm mean