It really depends what your strategy and team composition is and the nature of the encounter. For example, I needed to put together a Water team that could handle Ultimate Rahab on Auto-battle. Mordred turned out to be best-suited for this because she could extend the boss's overdrive meter and apply a string of debuffs that were less likely to be resisted, all while spamming Chaos Magic on cooldown. Other souls were too timing/RNG dependent and the team had to be micromanaged, which would be a huge time sink over the course of 100+ battles. Siegfried was great for ending the battle quickly in manual mode, but that still took longer than the 30s I spend setting up battles now. But on a boss that resisted or cleared debuffs Mordred would be useless. It's all situational.