FFI - FFOrigins. Still keeps the original difficulty but gives the UI a much needed overhaul
FF2 - FFOrigins, see above
FF3 - DS, though they changed enough that you could knock points against it for being too much of a departure
FF4 - DS, sorry to say this is just so much better than any of the SNES versions
FF5 - Fan-translated ROM.
FF6 - SNES Original
FF7 - PC Version (double the resolution)