Quote Originally Posted by The Devourer Of Worlds
In addition to this, FFV introduced new jobs and abilities that would form the basis of classes and Limit Breaks in later FF games. It also had the first truly complex plot device: The Void.
FFI technically defines the classes and everything else brances from those primary stems. FFIIIj also had the Void, but not in the psychadelic wierdness as it is presented in FFV.

As far as the most important one, besides FFI... *struggles* I'd have to... say...
FFVII, for reasons already mentioned, but I wouldn't go so far to say that FFVII had the most developmental/character driven plot, that's where FFVI/X reign supreme. If you'll excuse me I have to go slit my wrists now. FFIV is a very close runner-up for most important.

Well, FFV had all four elemental crystals. FFIX had one crystal that isn't even explained at the very end of the game. Kind of a cop-out crystal...but whatever
FFIX has the Elemntal Temples and fiends, but no Crystals were inside. I agree that FFV is the last one where the Crystals mattered, or we can even see they existed in. If there ARE Crystals in FFIX, then they don't resemble the "we balance the world and all Greek Science is dependent on us" type assertion that FFI/III/IV/V make them to be. Did I mention Greek Science and philosphy?