Susano'o is quoted by Sasuke as the "Ultimate Defense". We have seen that Itachi withstood Kirin, arguably the strongest and fastest technique in Sasuke's arsenal. We have seen Sasuke withstand attacks from various sources eg, when Gaara, Temari, Kankuro and Darui use a combination of 4 ninjutsu. Sasuke withstood it like a boss. Madara was able to fight on par with the god of shinobi, and even use the Susano'o as an armor for the Kyuubi. Every time I see Susano'o it only gets better.
The only way I can see Susano'o getting better would be if it was worn as a physical armor on a person, like how when Ichigo goes berserk in Hollow form. More of a physical presence and less like an ethereal one. Like this
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You can say who has the best one. You can argue which enemies can break through it, but never say it has been ruined. It is and always will be the ultimate trump card of an Uchiha when fighting an enemy deserving of seeing how bad ass it really is.
*Edit* Yes we have seen Sasuke use the sh*t out of it, but only because he needed to get full control over it. Now that he does, I hope it's back to using Fire Style and Chidori. But that's not up to me XD
OMG this is probably the most OP thing ever Itachi is a true badass who can solo a giant eight headed serpent on his deathbed"
