Here's the system we should adopt:
Roman numerals apply to the ACTUAL, chronologically correct, Final Fantasies.
Arabic based numbers apply to US releases.
i.e. FFI (FF1), FFII, FFIII, FFIV (FF2), FFV, FFVI (FF3), FFVII, FFVIII, FFIX, FFX, FFX-2 (LOL), FFXI, FFXII, FFXIII (FFAWESOME), etc...etc...