Maybe they could make an sequel to FFVII, that would be ... oh wait, what're you saying? Oh they already did, just forget what I was saying then.

Seriously though, If they were going to make a movie of any of the games I would vote for one of the three first games. Too little story to screw up, and they could even add elements to the movie (like having character developement) without destroying the storyline.

A prequel to FFV would also be great, but I would rather have it as a game than a movie.