If Shisui followed the 'Curse of Hatred'......
If Shisui followed the 'Curse of Hatred', he would've easily been in the Top 10 and Itachi would've been fodderized. Considering this is quite obvious, I would still get swarmed by ignorant Itachi fanboys on this thread. Although, I want you to know that I have deep respect for Itachi and he still remains a very talented and powerful shinobi. But anyway, I will explain the purpose of this thread as we go along.
Shisui Uchiha うちはシスイ
Shisui Uchiha, was a talented shinobi from Konohagakure. A noted member of the Uchiha Clan; one of the two most powerful clans of the Naruto World. He was famed for possessing the ability to control any being from his sharingan, Kotoamtsukami known as the most powerful genjutsu and as a result was feared for possessing such an ability. Shisui also had another ability known as Shunshin no jutsu in which he was able to transport himself to another location within a blink of an eye. Similar to Minato's Flying Thunder God jutsu, he was also able to use this really well. He was a dear friend of Itachi Uchiha's and revered as an older brother by him. Shisui initially followed the 'Will of Fire' ideal and sacrificed himself by entrusting his remaining eye to Itachi to protect the village, showing sincere devotion and loyalty after his other eye was stolen by Danzo.
However, during the time when he was about to give his sharingan to Itachi, there is no doubt that if Shisui was following the 'Curse of Hatred', he would've manipulated Itachi with ease rather than giving him his sharingan.
We all know that the 'Curse of Hatred' ideal is different to the 'Will of Fire' ideal and it's very clear since both ideals were passed down from the sons of Rikudo Sennin. The 'Will of Fire' descending from the Younger son who was sincere and chose to lead the world through Love and Peace while the 'Curse of Hatred' was descended from the Elder son who opposed his brother, the Younger son and intended to lead the world through Strength and Bitterness. As a result, two clans were formed that became feared as the two most powerful clans; the Uchiha Clan and the Senju Clan. After the sons of Rikudo Sennin were no longer in existance, two illustrious shinobi became the new leaders of the two mighty clans. Madara became the leader of the Uchiha Clan and Hashirama became the leader of the Senju Clan. Each clan contrasting and still carrying the ideals that were initiated by the two sons of Rikudo Sennin.
If Shisui Uchiha followed the Uchiha Clan's ideal, the 'Curse of Hatred' he would've been a more recognized shinobi, more powerful and likely to be more feared due to the ideal that he would follow. The ideal which focuses on choosing Might to be the key to Peace. He would be considered baleful and would probably put the same amount of fear in every person's heart as much as Madara Uchiha has done.
As for Kotomatsukami, it appears that there is no known counter for it. It is the most powerful genjutsu. No one would know that they have fallen prey to becoming the pet or slave of the user of such a powerful technique. It is far more powerful than Tsukyomi, another powerful genjutsu technique. The reason for this is simple, Tsukyomi requires direct eye-contact whereas Kotoamsukami does not. All that a ninja against the Tsukyomi needs to do is, just not take a glimpse or gaze into the eye that contains Tsukyomi. Unlike Kotoamsukami, it does not require direct eye-contact as we saw Danzo manipulate Mifune which was not officially said until Ao suspected. It is also theorized that Obito may have already borrowed Shisui's sharingan in the past to build his trust with Itachi, please see this for more info: Obito already used Shisui's eye!?
To conclude, if Itachi and Shisui were to fight, Shisui would win the match before it would even start if he followed the 'Curse of Hatred'. And thanks to the manipulation powers of Kotoamtsukami, Itachi would be under Shisui's control and Shisui would have access to Susano, Totsuka Sword, Yata Mirror and a bunch of other things like Amaterasu and Tsukyomi. Lucky for Itachi and the rest of the shinobi, Shisui had to follow the 'Will of Fire' ideal and make an honorable sacifice to protect the village from within the shadows in which he also did indirectly. When Itachi escaped the Edo Tensei and ended the Edo Tensei, it was all thanks to Shisui through his sharingan. Did anyone realize this?
EDIT: I forgot to add, if Shisui were to somehow obtain Senju DNA(maybe if he were to meet Obito or Orochimaru), he would be essentially invincible and would easily be in the Top 5 nevermind Top 10