The Final Fantasy games are many.  There are even more ports and revisions, some good and some awful.  Here is a list of (arguably) some of the best editions available for most games considered /vr/.

FF1: FF Origins on the PS1. Anything after just shits on it enough to make the extras worthless.  A patched NES version is also a grand edition worth playing and is better in some respects.  Stay away from GBA and later.
FFII: PSP version. Every revision fixes something.  If you're not in love with the looks try PS1.  The NES version is busted as fuck either way though.
FFIII: Pick. Both versions are extremely different but sure aren't any easier.  PSP diesn't have the DS' WiFi requirement for the strongest stuff. The original NES also has fan translations, try experiencing both if you want.

FFIV: Depends on how you want it.
>purest
PS1. Closest you're getting to original without amateur translations.

>most content
PSP has a nifty party arangement and final dungeon for the original plus TAY. I just wish it had graphics options.

>hardest
DS for many reasons. Also has an NG+

GBA is has the same postgame as PSP with more classic graphics.  Sound restoration exists, but there's nothing for the colors.  SNES exists but it either has an awful official translation and cut content or worse fan-translation.

FFV: GBA.  Best script, most jobs,and a neat postgame.  Sound Restoration exists, but no color fixes do.  Stay away from the PS1 version like the plague.
FFVI: http://vrshares.net78.net is probably the closest you're getting to a "best" as a lot of patches came out for it from color and palettes to restoring cut content.  it also has extra content.  Many like the original but it's buggy as hell and has a love-or-hate localization.
FFVII: PC has translation fixes but has the same localization. If no mods are wanted (there are a lot, including a complete relocalization/translation) it doesn't really matter much.
FFVIII: PS1. PC doesn't have as much changes.
FFIX: PS1. Has no other version.
FFT: PSP is good but patch that shit and find a bro.  If you don't have/don't want to do either, PS1 is perfectly fine.   You can even put the newer translation on it.