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.  It does however have loadtimes, which makes it slower than all the other options automatically as most of them have none.

>The most content
PSP has a nifty party arangement and final dungeon for the original plus The After Years and Interlude (which is exclusive to PSP). Only thing missing are graphics options, but some like them.

>Hardest
DS for many reasons. Also has a New Game+ with a ton more extras that are only for it and a completely new translation.  This can feel extremely different compared to the others, because of the entirely different graphic style which isn't unlike III's DS/PSP version.

GBA is has the same postgame and ability to make your own final dungeon party as PSP with more classic looking graphics, but doesn't have TAY or Interlude.  Sound restoration exists, but there's nothing for the colors.  SNES exists but it either has an awful official translation, cut content, and easy as shit, or it has an even worse fan-translation than the official with all (make that most cut content; the translators actually censored some of the text) the cut content there.

FFV: GBA.  Best script, most jobs,and a neat postgame.  Sound Restoration exists, but no color fixes do.  The SNES version also has a translation but 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 and an (arguably) better and expanded script.  Many like the original but it's buggy as hell and has a love-or-hate localization.
FFVII: PC has a few translation fixes but has the same localization. If no mods are wanted (there are a lot, including graphic updates and a complete relocalization/translation) it doesn't really matter much.
FFVIII: PS1. PC doesn't have as much changes.  The PC version requires replacement files to play the original music, whereas Square fixed this for VII.
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 great.  You can even put the newer translation with bugfixes on it.