Lights, Cameras, ACTION!!!

Now I can't imagine anything but Fran doing all of Beyonce's random epileptic-fit dances.

FFVI, definately
Maybe a FFX film would do, too.

FFV, it has a really basic plot and embodies the foundations of Final Fantasy, except it actually has characters. I think it could work well into a movie adaptation.

But really, I cannot imagine anyone would do it justice. I think a Final Fantasy movie would just be horrifying.

X and XIII could also work, but they'd have to be condensed quite a lot.