I think Chaos turned out to be the most satisfying. He was connected to a fund sidequest and he was one of the more interesting boss fights since he used Elementals and was the fight that removed the basic Fight command, meaning you couldn't just brute force your way with melee but also had to be careful with magic due to the Elementals. He's also one of the better Advanced Summons with a powerful attack and user-friendly build. I would almost argue he's the best summon to be honest since he's actually useful in battle due to his whole play-style being built around exploiting elemental weaknesses.