Roronoa Zoro is the swordsman of the Straw Hat Pirates in One Piece and one of the most important characters in the series. He's also the first person Luffy recruited on his journey to becoming the King of the Pirates.
Zoro is a very ambitious character and he aims to defeat Dracule Mihawk to become the World's Greatest swordsman for both Luffy and Kuina's sake. Over the years, he has advanced by leaps and bounds and acquired several power-ups to defeat the many adversaries that he has faced. However, some Zoro power-ups are certainly greater than others.
9 Sandai Kitetsu
The Sandai Kitetsu is a cursed sword that Zoro received during the Loguetown arc. Breaking all his weapons against Mihawk, Zoro was left with regular swords and Loguetown was the perfect opportunity for him to go shopping.
When he picked Sandai Kitetsu, Zoro tested his luck against its curse and managed to win it over. This sword, being cursed and graded in nature, offers Zoro a substantial power boost.
8 Yubashiri
Just like the Sandai Kitetsu, Yubashiri also offered Zoro a significant power boost. He received Yubashiri as a gift from the sword seller who he bought Sandai Kitetsu from.
The Yubashiri was known to be one of the 50 Wazamono Grade Swords. This sword was known to be light and easy to handle, yet had a sharp edge nonetheless. Unfortunately, Yubashiri ended up dying at the hands of Shu, a member of the Navy.
7 Breath Of All Things
Zoro's fight against Mr. 1 forced him to evolve in combat. He learned a unique power here, known as the breath of All Things. This power allowed him to tap into his spirit and determine which things to cut and which things to not cut. Since his opponent was a man made of steel, Zoro had to learn how to cut steel in this fight and that is precisely where Breath of All Things came into play.
In essence, the Breath of All Things is very similar to Ryuo, a form of Haki that was prevalent in the Wano arc. Zoro tapped into a rudimentary form of Ryuo and managed to cut Mr. 1 during this fight.
6 Shusui
In the Thriller Bark Arc, Zoro advanced his skill even further when he fought against Ryuma, his very own ancestor. Ryuma was in possession of a black blade named Shusui. This blade belonged to the 21 O Wazamono swords and it drastically increased the attack power of Zoro's weapons.
The slashes produced by this blade were much bigger than the ones produced by all other blades and this was evident during his fight against Oars.
5 Asura
Asura is an incredibly powerful ability that Roronoa Zoro possesses the origins of which are unknown to this day. Essentially, this ability allows Zoro to produce two heads, four arms, and four legs, which allow him to wield the deadly Nine Sword Style.
Essentially, Asura triples Zorro's attack power and makes him stronger than usual. He used this skill against Kaku in the Enies Lobby arc and also against Kaido in Wano.
4 Observation Haki
During the 2-year timeskip, Zoro trained under Dracule Mihawk and learned about the essence of swordplay. He also focused quite a lot on Haki training and managed to grasp the concept of Observation Haki pretty well.
While not as good a user of this skill as Sanji or Luffy, Zoro can use Observation Haki very well. In his fight in the New World, he has made extensive use of this ability to sense his enemies and to be vigilant at all times.
3 Armament Haki
Zorro also managed to learn Armament Haki under the tutelage of Dracule Mihawk. With Armament Haki, Zoro managed to coat himself in armor that increased both his offense and defense powers.
Zoro's Armament Haki is extremely powerful and he has displayed that during his fight against Pica and even against stronger adversaries, such as King. Zoro's affinity when it comes to Haki is the Color of Arms and this is where he has the most ceiling to grow.
2 Enma
Zoro received a powerful sword that once belong to Kozuki Oden, named Enma, in the Wano arc. This blade belongs to the 21 O Wazamono swords and is believed to be exceptionally difficult to master.
Enma sucks the Ryuo of its wielder, meaning not just anyone can wield the sword. To fully bring Enma under control means that one is a very capable swordsman, worthy of wielding the blade of Kozuki Oden. After wielding Enma, Zoro's swordplay increased severalfold and he stated that he was stronger than ever before.
1 Conqueror's Haki
The Color of the Supreme King, also known as Conqueror's Haki, is found within very few individuals in the world of One Piece. Zoro is one of these people and he tapped into this skill during the Wano arc.
Zoro used Conqueror's Haki to coat his swords while fighting against Kaido and managed to scar him. Later, he gained conscious control of the infusion of this skill in the fight against King and defeated him as well.