Eh, I like the higher difficulty of the PS1 remake but I think everything else in that release is inferior and there was quite a bit broken in the PS1 and any earlier releases.
So probably the PSP version is best "overall." That version does have the GBA's DoS epilogue/extended story, right?
For the record, what pushed me over the edge of liking FFII was that status effects are absolutely deadly for both you and the enemy (Confusion in Tower of the Magi, hello!), and positive and negative effects used by you are useful/mandatory. And even things like Swap, Drain, and Osmose, which I don't really use in many other Final Fantasy games, are almost imperative for long-term survival.