Personal picks of the best possible options/choices:
1. Original NES on a front-loading NES on a CRT TV for classic experience OR the ROM on a NES emulator on your PC OR PSP ISO on PPSSPP emulator OR GBA ROM on VisualBoyAdvance emulator. If there is one thing SE does, it is to port FF onto everything so many there are many choices for the original.
2. PSP ISO on PPSSPP Emulator.
3. 3D version: PSP ISO on PPSSPP Emulator OR non-3D version is 'Final Fantasy III Pixel Remaster' on PC with font correction patch as it's the only HD version of FF3 that is non 3D.
4. 'Final Fantasy IV: The Complete Collection' on PSP ISO on PPSSPP emulator.
5. GBA ROM on Visual Boy Advance Emulator OR PC version for best visuals at the cost of it being a butchered direct port of a iPad touch screen game... to a PC.
6. GBA ROM w/ multiple patches applied correcting color brightness over-saturation, sound restoration, and bug fix patches on VisualBoyAdvance Emulator OR PC version for best visuals at the cost of it being a butchered direct port of a iPad touch screen game... to a PC.
7. PC version through "7th Heaven" launcher. I wrote all about it here:
https://www.ffxiah.com/forum/topic/55377/7th-heaven-ffvii-pc-launcher-w-1-click-hd-mods/.
8, 9, and 10. I'm gonna guess HD Remaster versions on PC. I have 8 and it looks as good as it probably can. There is a modding community that was doing amazing work with 8 too (much like the love 7 was getting), but I didn't keep up with it.
11. Japanese client hooked up to a PS2 viewed on a CRT TV for obvious reasons.
12+. Out of my wheelhouse of expertise.
My specific personal choices:
1, 2, 3, and 4 on PSP emulator. 5 and 6 on VBA emulator. 7 through '7th Heaven' launcher. 8 as "Final Fantasy VIII Remastered" on PC. 9 as original PS1 ISO via ePSXe emulator. 10/12/13/14/15+ I never played.