The best writing in the game is hands down, as others have mentioned, FFVI, IX, and XII. I'd say VII and X are at the top end of the rest of the pack. Anything before FFIV is pretty schlock by simple virtue of being before there was really a focus on strong storytelling, and I'd say IV and V were comparatively weak next to VI (especially IV's habit of having characters "die"). The only games I really can't stand from a writing or storytelling angle are FFVIII (for completely losing direction after a very promising first act) and FFXIII (for totally dropping the ball on a potentially interesting conflict by using the power of love to save the day).