I quite enjoy XII's system myself. I thought it was engrossing and fun while it lasted. Though I do think in the end it was nothing more than the RPG of "do this when that", a factor it highlighted. It took a new approach but wasn't new at the end of the day. Heal when hurt, antidote when poisoned, cast fire when the enemy is weak against fire, that sort of thing. I've played too many RPGs to be too impressed with such things anymore. Which is why I've largely drifted away from the genre recently.

Anyway, I've only played IV and up, so I'm going to say IV's.