Though most members of the Uchiha clan will eventually develop a Sharingan, few will manifest its more powerful form, the Mangekyo Sharingan. Known as the "heavenly eyes that see the truth of all of creation without obstruction" the eye can only be activated when the user experiences a traumatic event involving the loss of a loved one. Tragically, this led some people to force the issue and try to eliminate their clansmen to get a Mangekyo, and it also led to a lot of misunderstandings about the true nature of the Uchiha clan.
Mangekyo users retain all the abilities of the Sharingan, including Izanagi and Izanami. They also gain new abilities, which vary depending on their user. No two users are the same, but all are powerful. Who are the strongest Mangekyou Sharingan users? There are plenty of great contenders, including Itachi, Sasuke, Obito, Kakashi, and more.
Vote up the Mangekyo users whose powers you most admire, and vote down those who aren't all that impressive.
Shisui Uchiha was an ANBU, as well as Itachi's best friend. He developed a Mangekyo because he felt responsible for a friend's demise - not because he actually attacked him, but because he didn't help him at the last second out of envy. The guilt caused his eye to transform. This granted him both the ability to manifest a powerful Susano-o that could attack with chakra needles, it also gave him the ability to use Kotoamatsukami. This is a genjutsu ability that lets the user enter a target's mind and manipulate their thoughts so thoroughly that they have no idea they were ever manipulated in the first place. He hoped to use this to stop the Uchiha coup d'etat, but ultimately ended up sacrificing his life and entrusting Itachi with the future.50247A strong Mangekyo Sharingan?
Madara Uchiha awakened not just the Mangekyo, but the Eternal Mangekyo after swapping his eyes for his brother Izuna's just before Izuna passed away. He could manifest an incredibly powerful version of Complete Body - Susano-o, which possessed a katana so strong that the shock waves it generated could cleave a mountain in half. Also, his perceptive abilities let him immediately understand how other Mangekyo abilities worked - the second he got his hands on Kakashi's Mangekyo, he could use Kamui with no problem.59470A strong Mangekyo Sharingan?
After witnessing his close friend and relative Shisui's suicide, Itachi awakened his Mangekyō Sharingan. This gave him the power boost that he needed to eliminate his clan. Using his right eye, he could perform Amaterasu, a flame that burns forever unless Itachi chooses to extinguish it. With both eyes, he can use Tsukuyomi, a genjutsu ability that creates vivid hallucinations of collapsing and distorted time. Because he has two Mangekyo, he can also use Susano-o. His version of Susano-o comes equipped with the immensely powerful Totsuka Blade, as well as the nearly impenetrable Yata Mirror.65785A strong Mangekyo Sharingan?
After watching Kakashi eliminate Rin Nohara, Obito Uchiha's Mangekyo awakens. Because he gave Kakashi one of his Sharingan eyes prior to this, he can't use it to the fullest extent of its abilities. He can't manifest Susano-o, and he can only use the short distance version of Kamui. However, he manages to make great use of what he can do. He can make himself intangible, and teleport himself and others into an alternate dimension. He uses this ability for defensive purposes and is so successful with it that he barely ever has to engage in any sort of actual combat.
When his spirit temporarily inhabits Kakashi's body, Kakashi is able to harness all of Kamui's abilities, and generate a powerful Susano-o, which they use to help Sasuke and Naruto take down Kaguya.38145A strong Mangekyo Sharingan?