The GBA version of FF6 is pretty awesome. The sound is meh compared to the SNES and PS1 versions but there are some sound "Restoration" mods if you plan on emulating.
The PS4 version of 10 is identical to the PS3 version with slightly improved textures. It looks better but as far as content, it's exactly the same thing.
7 on the PS4 is flawless. If you plan on modding, the steam version might interest you but if you can't be bothered, you'll enjoy the little quality of life improvements that the PS4 version has over the PS1. (3x turbo, no encounters, no load times).
8 and 9, while a great alternative to their PS1 counterparts, are not as amazing to look at on the PS4. The models and interface received a nice "HD look" but the game backgrounds did not. It's not terrible but it's not great either. Also, note that the PS4 version of Final Fantasy 9 does NOT have the same multiplayer options that the PS1 did. They do however have the turbo, no encounters, no load times benefits.
So, if I were to name the “definitive” versions, in my opinion, for the games you mentioned, they would be:
FF6: GBA
FF7: PS4
FF8: PsOne
FF9: PSOne
FF10: PS4