IMO the best thing they do right is create a new story with a new cast of characters and new battle system with each game. It prevents things from getting stale by constantly mixing it up over the past 23 years. (aside from the few sequels and spin-offs X-2, Dirge of Cerberus & Crisis Core)
If it were the same characters this whole time for the past 23 years we would likely be bored.
Although I still wouldn't mind seeing a Final Fantasy VII remake. 
Top 5: (only played 7-13... tried playing the anthology 5 & 6, but couldn't get into them)
1) FF 7 (my first Final Fantasy game, my first PS1 game... was absolutely blown away, materia system was awesome, favorite music, favorite hero & villain)
2) FF 9 (was so much fun, great characters, great story, learning abilities permanently from weapons and items was cool)
3) FF 10 (story and characters grew on me, despite some annoying voice acting, turned based battle system added strategy)
4) FF 13 (got to say I'm addicted to the battle system, so fast paced and easy to manage and quickly use a lot of spells/attacks, wish the story was as good for me)
5) FF 8 (best mini-game ever in Triple Triad, I collected all of the cards in real life, didn't like junction system, never used any of my magic because of it, good story though)
