To me, FFV is quintessential Final Fantasy. It has all the gameplay aspects that can typically be thought to make an RPG a Final Fantasy game, in their most accessible form--levels, abilities that you can "equip" learned by accumulating AP, black, white, and blue magic (not to mention red!, sword, and time), summons, a couple "super bosses," and a pseudo-multi-stage final boss battle.

Storywise it's not overly complex, but it also unites many of the common storytelling elements in the series: playable character(s) of lost/secret origins/heritage, a villain who acts as a 'front' for the true villain, the world being upended by "cosmic" powers, crystals and the Void, etc.

Basically, if we have one FF to put in a time capsule to be opened in ten thousand years to give those future people an idea what FF was all about, it should be FFV.