It depends on what you are looking for. I love the PS1 versions of FFI and II as it retaines their original difficulty and magic systems. FFI is pretty mean in the beginning of the game but it actually get easier as you progress through it. In the SoS and PSP versions, they changed the magic system around and lowered the cost of everything which destroys the difficulty of both titles. Gameplay is kinda all they have going for them so nerfing the difficulty is a no-no in my book.

The GBA versions of FFV and VI are godly and have wonderful translations and pretty neat optional content.If you're new to the titles, I'd say the new stuff might have more of a fun impact on you than us old jaded fogeys.

IV is kinda the odd one. I personally love the DS version as I feel it gives an old classic a new era spin beyond that I like the PS1 version as it retains the game's original difficulty and strategies. The GBA has some nice features but once again it nerfs the difficulty of the game. If you don't want to deal with frustration and possibly seeing the game over screen once in awhile then definetly pick up the GBA version.