Pretty much the fact that it's the only series that can reinvent itself over and over again with overly positive results.
The series has built a legacy that has become synonim with RPG worldwide, which is nothing short of magnificient, given that the genre itself isn't such an acclaimed one.
The ability for each game to encompass breath-taking stories and create individual universes with tons of untapped potential is also something to behold.
Who can forget Chaos' last Requiem, the fight of the Rose, the will against the Dark, the legends of Mysidia, the overwhelming Void, the destructive power of Magick, the Planet's strength, the legacy of the Witches, the Crystal's memory, the Spiral of death, the devotion and struggle of a small nation against the titanic empires or the struggle of breaking free from the oppression of a unwilling Fate?
This is what Final Fantasy did and will continue to do right. Giving us, the gamers, stories, characters, gameplay and many other things, that will continue to inspire us and give us a sense of being involved into something epic, something that will have no definitive Finale.
Current PC Build
CPU - i7 8700K 3.7 GHz (4.7 GHz turbo) 6 cores OC'd to 5.2 GHz with Watercooling (Hydro Series H110i) | MB - Gigabyte Z370 HD3P ATX | Gigabyte GTX 1080ti Gaming OC BLACK 11G (1657 MHz Boost Core / 11010 MHz Memory) | RAM - Corsair DIMM 32GB DDR4, 2400 MHz | PSU - Corsair CX650M (80+ Bronze) 650W | Audio - Asus Essence STX II 7.1 | Monitor - Samsung U28E590D 4K UHD, Freesync, 1 ms, 60 Hz, 28"