The strongest of these swords are the Wado Ichimonji, Shusui, and Enma. 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.
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.
Zoro usually carries three swords with him to perform the three-sword style anytime. So currently, he has the Wado Ichimonji, Kitetsu III, and Enma with him.
Actually not just 1, he can eat 3 devil fruits. Yes.
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.
Wado Ichimonji is the first named sword that Roronoa Zoro carried in One Piece. It was forged by Shimotsuki Kozaburou, a person from Wano Country. This sword once belonged to Koushiro, the dojo master of Zoro from Shimotsuki village, and was meant to be inherited by Kuina, his daughter.
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.
Thus, Zoro's power grows threefold, as he can use his nine swords to slice an opponent thrice more than usually, and Zoro tends to do so by rushing past his foe at high speeds. Zoro developed Nine Sword Style before the two-year timeskip, first showing use of it during his intense duel with the CP9 agent Kaku.
On his journey to become the greatest swordsman (topped only by his mentor, Dracule Mihawk), Zoro has mastered multiple sword/blade-styles; from the three-sword (Santoryu), to both two-sword (Nitoryu) and one-sword (Ittoryu).
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. While the sword is certainly not a Supreme Grade sword yet, it also hasn't blackened.
The twenty-one Great Grade Blades (大業物, Ō Wazamono?) are among the highest-quality blades in the world, second only to the twelve Supreme Grade swords. These blades are well known for their incredible power and outstanding balance in strength, sharpness, and durability.
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.
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.
That said, there is one crew member who stands above the rest in terms of loyalty to Luffy: Roronoa Zoro. The Straw Pirates' first mate has proven his loyalty on numerous occasions throughout the series, even putting his crew mates' well-being above his own life.
Fearing its curse, he refused to sell it until Zoro took a liking to Sandai Kitetsu because of its cursed nature. Zoro courageously tested his luck by risking his arm, extending it against Sandai Kitetsu's falling edge, ultimately keeping the sword after his luck prevailed over the curse.
Wano Country Arc
After Luffy was defeated by Kaidou, Nidai Kitetsu was presumably confiscated by the Beasts Pirates and was at Udon. After the take over at Udon, the sword was retrieved by Kikunojo and returned to Sukiyaki, at which point Zoro was able to receive confirmation that it is indeed a Meito.
As one of the masterpieces of the legendary Wano swordsmith, Shimotsuki Kozaburo, classified as one of the 21 Great Grade Swords, Wado Ichimonji is among the world's most powerful swords, second to Enma.
Crafted by none other than the legendary swordsmith Shimotsuki Kozaburou, Enma is one of the 21 O Wazamono grade swords to exist in the world of One Piece. Kozaburou forged his sword when in Wano and to the day of his death, he remembered this blade as his greatest creation.
The Wado Ichimonji is a sword of great personal importance to Roronoa Zoro, and it was once Kuina and her family's sword. It is also one of the 21 O Wazamono grade swords.
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.
He uses the Nidai Kitetsu when he faces Basil Hawkins, to which Luffy threw a punch with the sword in his hand. The Nidai Kitetsu (trans. "Second Generation Ogre") is a cursed sword that supposedly brings a horrible death to its users.
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.