Final Fantasy IV is the best game of the series. Period.
While its graphics are not up to snuff compared to more recent titles, the actual story, the whole reason RPG games are played, is note perfect. In this particular telling, the driving force of the story is our hero's quest for redemption.
The gameplay is fantastic, with 5 playable team members as opposed to three, a difficultly level that is hard enough for a veteran and yet easy enough for a novice to work into, great music, and a gameplay experience that lets you see everything the game has to offer without having to invest an unnecessary amount of time and replay.
Contrast this with other more recent Final Fantasy titles, and you see why FFII is a classic.
Final Fantasy II sticks closely to the general outline of Joseph Campbell "Monomyth". Campbell makes the case that all "great" stories share a basic outline and themes, and you can find these themes in every story from every culture on the face of the earth.
While some of the details might be a bit different each time around, the basic journey of the hero remains the same.
If you are willing to put up with SNES graphics, FFII is THE best Final Fantasy game of the series. You will not find a better example of a classic story & engaging gameplay anywhere else.