I know, this is a old topic, but I still want to share some of my thoughts on it =)
Now Yamato seems Godlike at first, but if you take the whole Story of Sparda into consideration, it makes sense and all other swords are just as Powerful in different ways.
First let us Talk about Sparda, the character: Sparda, after defeating Mundus, decided to seal his Powers away alongside the demon realm. But how was he able to seal ONLY his power, without sealing his whole body and person? That is where the 3 devil Arms come into play. How the Yamato was created is a mystery but DMC1 implied (with the Sparda costume) that Sparda used this sword in the human world after he sealed his powers. So MAYBE he forged this sword in the human world with his powers but it is technically not a part of him, like Devil Sword Sparda. That would explain why it looks so "mundane" and normal in design compared to his other weapons. he created it to split his powers from himself, creating the Force Edge (the physical Form of his powers) and the amulets (the seals in which his Powers most likely reside, since the Devil Sword Sparda can only be reforged with both amulets combined to release it).
During this Process or after it, Sparda probably also created the Rebellion as a backup plan. Even with Demon Realm sealed, it could still be possible that demons might return and his powers might be needed once more, so the rebellion was created in case this happens. If the seal to the demon realm should ever be broken, Sparda can use the Rebellion to combine himself again with his old Powers, returning him into his original demonic self.
That would explain why the Devil Sword Sparda was absorbed when Dante stabbed himself with the rebellion in DMC5, since this was most likely what it was originally intended to do. This was actually already hinted at in DMC3, when Dante got stabbed with the Rebellion, which suddenly activated his Devil Trigger, and transformed the swords design a little. By being stabbed with it Dantes Soul has forcefull become one with his Devil Powers. I say forcefully because Dante seemed to never have accepted his demon heritage like Vergil did and thus never unlocked his power on his own.
In fact, it seems that the Yamato is actually the "weakest(!)" of the three by comparison. The Devil Sword Sparda is the true strenght of Sparda, with a Blade that can change Form to give access to varying attack mdoes, the Rebellion that can unleash the true potential of someone who forcefully unleashes their inner Power.
Yet the Yamato is meant to actually STRIP Power away, and is also used to SEAL things rather then unlocking power. However, it also comes with something that is probably unique to it (and that is HUGE speculation on my part, so not something that is actually to be considered canon, it is just my understanding of things): It refelcts the wielders TRUE Power. It is weird to see how differently Yamato used by each person that wielded it. In fact, Vergil is the only person (maybe besides Sparda) who was truly able to wield the sword. Dante used in DMC4 but he merely imitated Vergil, and it was not as effective as the weapon could potentially be (That was stated I believe in one of the Data Books IIRC). Nero didn't even used the sword REMOTELY as Vergil or Dante, he used to fire off Blade Beams (which neither Dante or Vergil ever did with Yamato) and uses just like a normal Sword. Even the Spirit of his Original DEvil Trigger from DMC4 used it as a normal sword.
This makes me think that outside of the fact that it can open portals and is used for sealing and splitting power away, it has not much Power of it's own, rather it simply draws out the full potential of it's wielder.
Vergil was just the one who got the most out of the sword, since he wielded, trained and mastered the blade to perfection.
That is just my take on all of that though.