For me, FFVI has too large a cast with too much going on for a 2-3 hour film. It'd be better as a mini-series, perhaps.
FFVII likewise. FFVIII is too ridiculous plot-wise. XII too political and probably too much to be condensed. FFXIII is probably the best shot.
I think films vaguely based on the plots of the games would work better than films trying to accurately match the plots and characters.