And more remasters for SaGa as well, though they were just tossing out titles, no specific announcement yet.
I&II
Origins for PSX. Subsequent remakes for GBA, PSP, etc. skew the difficulty and balancing to the extremely easy end.
III
DS remake is good and is the basis for PSP/Steam ports, but
FC with fan translation is also good if you'd prefer the original artstyle (and peculiarities of the battle/job system)
IV
PSP version gives you updated translation and presentation but within 16-bit conventions. GBA is also okay; DS remake (basis for Steam version) is enjoyable but basically a different game unto itself.
V
GBA version is the one — the first official English translation of the game. Steam is okay if you can put up with compromised presentation.
VI
SFC version with fan translation will give you the best presentation and sound. I'm going to be playing the
"Ted Woolsey uncensored" version sometime later this year, which has been recommended elsewhere here I believe. Steam, like with V, is playable but compromised.
Of course, the potential for an Anthology release announced at E3 would make everything straightforward — if they don't just repeat what they've released on Steam that is!