Out of the ones I've played, II. I applaud its early attempts at storytelling and a unique leveling system, but it falls short on just about every level. The story is still very simple by modern standards, the world is confusing yet monotonous and boring, the battles are dull, leveling up is stupid, guest character suck after the first few hours, and the soundtrack is not as memorable as the original's or latter games.
Mind you, I haven't played 5, 8, 9, 11, 13, or 14, so I can't be a good judge of this.