Does there have to be one? They all have there plusses and they all have some weaknesses comparatively. I do have a clear line of favorites (starting with FFVIII) but you don't have to have such an order. As I consider all FFs to have pretty similar gameplay what's important to me is the story the game tells, the characters and the feel of the game. So based on those principles your favorite would be FFVII. But what's important is they're all wonderful games you can enjoy.