Zoro is one of the main characters of One Piece and among the most reliable members of the Straw Hat Pirates. As a master swordsman, Zoro's strength is second only to Luffy and his incredibly ambitious goal sees him target the throne of the World's Strongest Swordsman. To get there, Zoro needs to put in a lot of work and he has defeated countless enemies standing in his way.
Zoro has been in countless battles and received many scars as well. Along the way, he has improved his strength by overcoming the adversities that he has faced. While there are many battles that he has been in, there are some that make him shine more than others.
6 Vs. Pica
Zoro fought against Pics towards the end of the Dressrosa arc of One Piece. Pica was one of the higher ranking Officers of the Donquixote Pirates. With the powers of the Ishi Ishi no Mi, Pica could assimilate stone and control even large plateaus, which is an incredible feat.
Zoro clashed against Pica and in this fight, he demonstrated his Armament Haki for the very first time after the timeskip. Zoro showed how far he had come following his training with Dracule Mihawk and ended up completely overwhelming Pica. In this fight, Zoro wasn't pushed to even half his strength, which just goes to show how powerful he was following his training with Mihawk.
5 Vs. Baroque Works
On Whiskey Peak, Zoro fought against 100 members of the Baroque Works all by himself. When the group tried to lull the Straw Hat Pirates in and assassinate them, it was Zoro who managed to see through their plans, along with Nami.
However, Zoro took on the challenge of fighting them all alone. Later on, he also had to clash with Luffy, as his captain was upset at the treatment he gave to the people who supposedly welcomed them to the island.
4 Vs. Kaku
Zoro fought against Kaku during the Enies Lobby arc. Kaku was introduced to the fans during the Water 7 arc where he posed as a shipwright of the Galley-La Company.
In time, Kaku was revealed to be a member of the CP9, a secret intelligence agency of the Government that specialized in assassination. Zoro took him on in order to rescue Robin. The fight was incredibly difficult for him, however, by channeling his inner powers, he was able to win. Zoro had to tap into the power of Asura to overcome him, however, he did so in quite some fashion.
3 Vs. Ryuma
Ryuma is another incredible foe that Zoro ended up fighting in the Thriller Bark arc. This fight is doubly special, given that Ryuma is actually an ancestor of Zoro. In Thriller Bark, Ryuma was a zombie brought to life with the powers of the Kage Kage no Mi of Gecko Moria.
While not as strong as he was in real life, Ryuma was still a menace, capable of being a general zombie-level fighter. His fight against Zoro was extremely intense and short-lived at the same time. Since they were both high-caliber swordsmen, the battle ended quickly and it was Zoro who ended up taking the victory here. After defeating him, Zoro also earned Shusui, the prized possession of the land of Wano, from him.
2 Vs. King
Zoro ended up fighting against King in the Wano arc of One Piece. This fight remains to be one of the hardest that he has ever fought to date. Before the fight, Zoro was in no condition to take on King in battle, however, thanks to a magic Mink drug, he was restored to full health for a temporary time period.
During this time, Zoro did everything possible to defeat King but initially, he struggled quite a lot. King proved to be a menace for Zoro to deal with, however, all it took for him to defeat King was to master Enma. This sword has tremendous properties as it allowed him to channel his Conqueror's Haki to the fullest. With this newly unlocked power, and the fact that he was able to figure out King's weakness, Zoro powered through and managed to defeat King.
1 Vs. Mr. 1
Zoro versus Mr. 1 remains to be his very best victory to this day. This fight was incredibly difficult for him and it tested him on both a spiritual as well as a physical level. Mr. 1 was incredibly powerful, as is expected of the right-hand man of Sir Crocodile. His steel body meant that Zoro couldn't cut him at the time and, thus, he suffered terribly. Zoro was in a near-death situation during this clash and it is only then that he received a power-up that allowed him to cut through steel.
Zoro managed to level up during the fight and his newfound abilities allowed him to slice through the steel body of Mr. 1. That said, this fight still saw him bruised and battered and on the brink of death. Undoubtedly, it is Zoro's most impressive fight as well as victory to this date, in the manga, at the very least.