About to finish playing FF IV. So far I've played V, VI, and Chrono Trigger. Guess you can count IV as well, since the only thing left to take care of is the final boss. After this, I'm going to try Super Mario RPG.
None of the games I've played are quite as good as V. Another case where the most popular game in the series (VI/VII) isn't necessarily the best. There's no Hide in VI, no Mix, defensive mechanics are largely worse, offensive mechanics got gimped (Dualcasting disables Summons, you can use Summons once per battle with each character, whereas in V you could Dualcast Summons and use them pretty much whenever you wanted to), among other things.
If you guys want to give these games a try, please watch out for the following:
If you plan to play IV's SNES version, be aware that the US version (called FF II) removed offensive items and status effect inflicting ones. Needless to say, IV Easy Type does the same thing. If you really want to play the best version on the SNES, you need to play the Japanese version. Either learn Japanese, have a guide at hand, or use a translation patch. If you want to play the GBA port (guess this also goes for PSP), you'd have more content than in the SNES version, but you'd need to farm for items more, as the item duplication glitch was fixed.
For V, just play the US GBA version. It has more content than the SNES version, and there's even a pretty beneficial glitch that IIRC is not present in the original, which saves you a lot of trouble when mastering the Jobs. The GBA EUR version fixed this, so stay away if you can.
For VI, I guess you can just go for either version of the GBA port. As usual, more content than in the original. Only noteworthy differences are the Vanish/Doom glitch (Vanish overrides death immunity, so Doom works on almost, if not every single thing. For those curious, Doom is a multi targetting Instant Death spell) and the Evade bug (normal Evade% is bugged and does nothing, but the Magic Evade% value determines the rate in which you dodge BOTH physicals and magic, and I think maxing out one of those values is easier than trying to max out both, so the endgame becomes a bit easier if you've not been exploiting Vanish/Doom already). In other words, if you want to play the game the way it was meant to be played, go for the GBA port. US/EUR does not matter, I'm pretty sure.
For Chrono Trigger, get the DS version if you can. More content, and it's the only version where the item duplication glitch is possible, saving you some headache if you're not the grinding type. Steam versiod axed the glitch in question.