I would say SNES Classic for FF6 so you can get the good old couch experience.
But really, I'd say emulate FF1-6 on a raspberry pi or off a laptop hooked up to a TV. When you're grinding, unplug the laptop and binge TV to take the grinding edge off.
FF1 - Read a few guides or watch some strategy videos before starting. Don't pick a Red Mage for your party, it's a waste. The GBA version is easier. But I'd say a modern emu with savestates is fine.
FF 3&4 - Maybe try the DS versions, but I'd say use an R4 cart. Unlike the GBA versions which were a little easier, the DS versions are a little more grind-y. That's fine for 3 but FF2 in America (FF4) was beloved because it wasn't grind-y and had a solid story you could beat in a rental period.
So, activate some cheats on the R4 carts for the DS versions.
Besides that, the other big downside of the DS versions is the voice acting... "WWWHHHYYYY!!!!" oh Ceh-sill. (Cecile)
Like a lot of people, I really don't think many re-releases have done justice to FF6. The PS1 version is fine, that's about it. SNES Classic or emu is better though for save states. PS1 also had a cool intro in the artstyle of Amano, which is the only time (iirc) that his weird, cool artstyle is rendered in 3D.