If you're asking about magic & combat system, FFV is the best and the best main series FF with a job system.

You didn't ask about story, but for that one FFIV might have the best scripted, linear storyline out of all the original 2D FF's.

The rest are all good, but it depends on what version you play whether or not you'll like it. From what I can gather - FFI = Playstation One version (Final Fantasy Origins); FFII = Game Boy Advanced Version (Final Fantasy I & II: Dawn of Souls; FFIII = tie between NES and DS version; FFIV = Game Boy Advance version; FFV = Playstation One version, or Game Boy Advance if you don't mind handhelds, or the untenable D-Pad & Analog Stick on the Gamecube (if you have a GBA player for it). I like the PSone version better because of controls and the battle screen is like 30% cut out because of the GBA screen.