Well, for specifically the first six, these are my favorite versions:

FFI: PS1 version (retains original difficulty with option of easy, updated graphics and sound)

FFII: GBA version (extras, updated graphics and sound, better translation of names compared to PS1- Ricard, Guy, Minwu VS Gareth, Gus, Mindu)

FFIII: Famicom version (prefer sprites over DS models, like extra challenge compared to DS)

FFIV: PS1 version (prefer the SNES sprites, loading times aren't that bad, original difficulty)

FFV: SNES version (mainly the soundtrack is better compared to GBA)

FFVI: SNES version (mainly the soundtrack and some parts I think were translated better in the SNES compared to GBA)