I think what makes Final Fantasy so great and keep going where many other franchises have failed is that each one is a completely seperate adventure with new characters, new story and new world to explore. They each try to do things slightly differently and breathe fresh air into the JRPG genre (for better or worse). They'll change the battle system, the levelling system, the mythos with every iteration and this keeps people going back.
Even if you hated one of the Final Fantasy games, if you've enjoyed at least one Final Fantasy game, chances are you'll find another Final Fantasy game that you can enjoy. Final Fantasy peaked a while back for me (6 & 7), but what keeps me buying and playing them is the knowledge that each one is different and that they may suddenly hit the right formula for me personally. With such a large and critical fanbase it's very difficult to perfect the game 100% for everyone, but they do a fairly good job of it.
Not neccessarily my faves but worth mentioning for one reason or another:
FFVII: Love the cyber-punk world and story. The first 3-4 hrs in Midgar being a story intense but streamlined experience really helps to get into the story.
FFVI- Great story, some classic characters with some very memorable moments.
FFX: Nice story, beautiful game
FFXII: Lots of potential in the story, freedom and a cool battle system
FXIII: Again, great battle system.
FFV: Job system was brilliant and evolved nicely from past games.
FFVIII: Tried something new and innovative with the Junctioning system.








