I have only played 2 Final Fantasy games and watched my little sister play Final Fantasy VII and Crisis Core. From those experiences, I say the new FF games have better gameplay, graphics, and good translations. The older FF games (9 and below if you want) had more subtley, an engaging story and deep cast of characters, and unforgettable music. I say unforgettable music because FFXIII, Lightning's game, had limited music. I only remember one piece because they kept using it over and over. The rest of the track except the Chocobo I forgot. For FFVII and VI, I remember the music clearly from Sephiroth, Aerith, Terra, to Kefka.
Subtlety and a lot of interpretation in FFVII and VI made the game more alive than FFXIII. Fans can continue to speculate and discuss making the replay value high. After beating FFXIII, I didn't feel much attachment to the characters or care about the story.
That's all I say. I wish I had more time to play video game. My siblings are lucky that they are not in college.
I wanted to play FFX for a while. I heard FFX-2 was pretty bad.