The greatest storyline, huh?
I'm gonna have to go with FFV & FFVII.

Final Fantasy V:
FFV has a very complex and intriguing story.
Ex-Death was straight out insane, and the semi-villain Gilgamesh was incredible vicious. The story was ever so great, it had me captured the whole time with its amazing atmosphere and it's constant unpredictability. Overall I really think that FFV's story has been way too underrated throughout the years.

Final Fantasy VII:
Shame on you, if you don't know it by now...