Avenger and Argonaut seem to be the most powerful but you have to take in count that it depends on the situation.
Avenger:
Advantages: 1. Is perfect for monsters. 2. It can increase its power with hate. 3. It can be used for multiple attacks without waiting to charge. 4. It can be used to increase the power of magic and/or weapons. Disadvantages: 1. It damages the weapons the more it is used. 2. It is useless against humans.
Argonaut:
Advantages: 1. It can be powerful enough to instakill an opponent far more powerful than the user. 2. It can be charged while moving. 3. It can be used against both monsters and people. 4. It can be used to increase the power of magic and/or weapons. Disadvantages: 1. It requires more time to charge for more power. 2. If the user lose concentration or gets hit, it loses the charge. 3. The longer the charge is the more the weapon is damaged.
We have seen that a single Firebolt from Bell with 3 minutes of charge was able to blow up like nothing the head of the Goliath Level 5 with a skin so strong that even the Juggernaut is unable to damage, even being Firebolt a very weak magic and Bell being only a Level 2 and as a Level 3 with a charge of only 22 seconds he was able to make a lot of damage to Dix who was a Level 5 with a single punch. So we can say that with a full charge of 4 minutes and him being a Level 4 it doesn’t exist a single adventurer or monster that could survive, but still is the problem of having to charge. On the other side, Avenger is able to increase the power of Ais enough to be one level stronger as she was able to fight against Revis who had the power of a monster Level 7. So it depends of the situation, if you need to increase your power more or less one level and quickly against a monster, then Avenger is perfect, but if the enemy is far more powerful than you , then Argonaut is the best option.