FF6 Steam Version

I think practically the SNES version is the most moddable. As for its limits, stuff like VRAM is already pushed to a certain limit in general. Some stuff can still be expanded with proper tools (example FF6LE+) but at the same time if you want heavy changes you have to be able to rewrite larges portions of sometime obscure code and maybe modify code new tools.

The PC version might have certain limits the SNES version has since it seems to relies heavily of the original data structure. Little to nothing is know about its code also.

That been said even if there was a PS4 remake nothing stops you of trying to test new boundaries on the SNES version. There's no comparison to do, everyone gets his satisfaction from something different, me it's in general tools development and coding, which the PC version (and still the SNES one) offer great opportunities.