The Susanoo is the mightiest technique of the Mangekyo Sharingan. The Susanoo requires its user to use both eyes together to manifest a large, ethereal warrior capable of great defensive and offensive capability. Susanoo's weapons vary between users, but its defensive power, even in a fully complete state is limited for all users. So far, Madara Uchiha's version of the technique has the greatest defense abilities displayed. For those who have only activated the standard Mangekyo Sharingan, this technique acts as a double-edged sword as it devours their life force gradually and will eventually kill them, while also causing great strain on the eyes and body. However, those who gain the Eternal Mangekyo Sharingan are at no risk using this technique, and it's shown to be more powerful than the standard version.
Example user
Madara's Susano'o's ribs.
Madara Uchiha- Madara's Susano'o is the most powerful version of Susano'o to date, having several different forms with different power, but similar weaponry. Like all users, his initial use of this technique was in just a ribcage version and thus his weakest form, though still quite impressive as it withstood the Raikage's massive strength, which had damaged Sasuke Uchiha's at that stage easily, without receiving any damage, and similarly took a direct hit from Naruto Uzumaki's Sage Art: Giant Rasengan without a scratch.
Madara's complete Susano'o.
His more complete versions all sport unique, very powerful swords with different power. Weaker versions were able to smash through the defensive efforts of two Kage with ease in one attack. More complete forms of Madara's Susano'o he was seen using in conjunction with his Rinnegan to summon giant meteorites.
Madara's stable Perfect Susano'o.
Madara's final and greatest form of Susano'o, the Perfect Susano'o, is a skyscraper-sized ethereal warrior as big as Kurama, the Nine-Tailed Fox. Its initial form is unstable and its appearance shifts like flames, but it's still a head-and-shoulders taller than Madara's meteorites. At Madara's command, this Susano'o solidifies, taking on the appearance of a giant samurai wearing a mask, with four arms, two plated and two not, armed with two massive katana wielded by its front arms. This form is near-invincible and the only thing known to have broken it, albeit incompletely, is Hashirama's giant wooden Buddha. He stated this Susano'o's power is equal to a tailed beast and is his full strength, overwhelming even the Five Kage's ultimate attack without any effort.