Enma is a ō wazomono blade like shusui but shusui is already a black blade which was turned by Ryuma so it could have reached it's potential while Enma is still a normal blade and if Marimo turns it into a black blade, It will become a Saijo ō Wazomono (12 supreme blades) hence becoming stronger.
Either a) by the same logic Shusui is technically a grade weaker than Enma, Zoro would still have a weaker blade in Enma than Mihawk's Yoru and WB's Murakumogiri, both blackened Saijo O Wazamono swords, which should be stronger than blackening an O Wazamono sword like Enma.
What is Zoro's weakest sword? Out of the swords he currently owns, the Sandai Kitetsu is the weakest blade in his arsenal.
Shusui may be Zoro's favorite sword, but it's still weaker than his other sword Enma, this is due to Enma being a Great Grade Blade, putting it a class above Shusui. That said, it can keep up with Zoro's other swords, Sandai Kitetsu, Yubashiri, and Wado Ichimonji.
Wano Country Arc
Shusui was later stolen by Gyukimaru, who returned it to Ryuma's grave. In order to have Shusui remain in Wano Country, Kozuki Hiyori offered her own Great Grade sword, Enma, to Zoro as a replacement for Shusui, a proposal that Zoro accepted.
In exchange, she offers to offer him her own sword, Enma, which was bequeathed to her by her father Kozuki Oden. After a brief hesitation, Zoro accepted the proposal. And this is how Zoro lost Shusui, which is now located in Wano, as part of Ryuma's grave.
According to Tenguyama Hitetsu, Enma didn't get a chance to turn into a black blade, but under Zoro, it'll definitely turn into one.
Wado Ichimonji is Zoro's most treasured weapon as it was been given to him by his master after his childhood friend and greatest rival's untimely death. It's become both a piece of prized equipment and a reminder of their promise to become the strongest swordsman.
Wado Ichimonji is one of the strongest swords in the world and was crafted by a master swordsmith, Shimotsuki Kozaburo. Kozaburo crafted the sword and left Wano, settling in East Blue a few years later. Zoro inherited the sword from his master after vowing to become the strongest swordsman in the world.
The strongest technique that Roronoa Zoro uses in the Dressrosa arc is a Three Sword Style ability known as the Billion-fold World Trichiliocosm.
Later, in the fight against King, Zoro used Conqueror's Haki to tame Enma completely. Eventually, Zoro managed to coat all his swords in what appears to be the advanced form of Conqueror's Haki, making him one of the strongest swordsmen in the story now.
Dracule Mihawk is Zoro's biggest rival, being the man who holds the title of the World's Strongest Swordsman. Mihawk is extremely skilled with the blade and his swordplay is described as very aesthetic.
It's entirely possible Mihawk noticed through their duel and training that Zoro suffers from ocular dominance in his left eye. That is to say, the left eye is stronger than the right. As a result, maybe Mihawk scarred the left eye shut and instructed Zoro not to open it until his right eye was trained enough.
1) Sasuke Uchiha
He obtained the Rinnegan and had the Mangekyo Sharingan. Not to mention, being Indra's reincarnation, he also had Indra's Arrow, an extremely powerful technique. With his expert use of lightning, ninja tools, and jutsu, Sasuke could easily overpower Shisui.
Minato was capable of taking down entire armies on his own, and he was more than capable of beating Shisui.
Shisui mastered the Sharingan, which made him a very dangerous opponent. In addition to his dojutsu, Shisui was proficient at using the Body Flicker Technique. His mastery of the Body Flicker earned him the nickname "Shisui of the Body Flicker." He became famous and ninjas throughout the world feared him.
Zoro originally received the Wado Ichjimonji from Koshiro. Initially, this sword was supposed to be passed down to Kuina, a girl at Zoro's dojo who defeated him 2001 times. Due to her untimely death, Zoro took possession of the Wado Ichimonji instead.
It was formerly wielded by Kozuki Oden and was passed on to Kozuki Hiyori, his daughter, after his death. This sword was then given to Roronoa Zoro in exchange for Shusui.
Despite having lost its use, Zoro kept the remains of the Yubashiri in its sheath and on his waistband from then on until he found an appropriate resting place for it.
More than anything, Wado Ichimonji holds a great deal of significance to Zoro, though he'd allow others to examine it, he refuses to ever give this blade up to anyone.
Zoro developed Nine Sword Style before the two-year timeskip, first showing use of it during his intense duel with the CP9 agent Kaku. After being overpowered by Kaku's ferocious attacks, Zoro awakened Nine Sword Style, which allowed him to overpower Kaku's most powerful Rankyaku and cut the CP9 member down.
As one of the 50 Skillful Grade Swords, Yubashiri was a rare and reliable weapon that served Zoro's unique fighting style well. It was destroyed during Zoro's fight with Shu on Enies Lobby, and was later left alongside the Rumbar Pirates' gravesite on Thriller Bark.
Monkey King: Enma (猿猴王・猿魔, Enkōō: Enma) was the personal summon of Hiruzen Sarutobi, the Third Hokage.
Zoro with Enma and his two other swords is stronger than Zoro with Shusui and his two other swords. Zoro with Enma basically gave him a buff with being able to cut things more deadlier with the power of Enma along with his Haki, but Enma wasn't in his full control like he was with Shusui.
Comparing it is like when Naruto summons Gamakichi or Gamatatsu, which are summons of the same species (family) as Gamabunta, which Naruto also manages to summon. Following that logic, we can conclude that Konohamaru could summon Enma.