Roronoa Zoro is the Straw Hat Pirates' vice-captain and a renowned swordsman who previously held one of the highest bounties of the rookie pirates (AKA the Worst Generation). With a goal to become the world's strongest swordsman and an aptly given epithet of "Pirate Hunter", Zoro has continued to hone his craft throughout all of One Piece, boasting tremendous growth as the series has progressed.
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). Before dedicating himself to the Santoryu style, Zoro specialized in Nitoryu, being the first style he adopted. As such, these are Zoro's best Nitoryu-style techniques.
10 Nigiri: Otoro
A staple of Zoro's Nitoryu style is his many Nigiri techniques (Nigiri translating to "Two Cutter"). The Nigiri stance involves Zoro holding his two swords parallel to one another and to one side, setting him up for an array of different follow-up slashes.
Otoro is one such Nigiri-based technique, which pairs well with the technique Toro. Toro ("Climbing Tower") sets Zoro up by slashing both swords upwards as he ascends with two air-based projectiles; he then follows with Otoro ("Reply Climbing Tower") by slashing downward in an opposite fashion to Toro. On its own, Otoro isn't the strongest form of Nigiri, but when used alongside Toro it makes for a better technique.
9 Taka Nami
Taka Nami is a technique that Zoro primarily uses for group-based combat, as it is capable of hitting multiple enemies at once. The technique sees Zoro jumping into the air and slashing his swords in a way that creates a circle of wind. Due to the degree of Zoro's slashes, enemies hit by the wind are easily knocked over.
An interesting fact about Taka Nami is that it was the first Nitoryu technique Zoro was seen using with the Straw Hats. While the technique is effective for crowd control, it ultimately does the least amount of fatal damage since it focuses more on disorienting many opponents rather than physically harming them.
8 Nigiri: Maguma
Like Otoro, Maguma is a technique that precedes the Nigiri stance. Mechanically, Maguma is a relatively simple technique: Zoro jumps upward and comes crashing down onto this opponent with both swords, hitting them head-on. Intriguingly, the technique strays from the play-on-words of the other Nigiri techniques having sushi-related names, as Maguma seems to play on "magma".
Zoro first used this technique against Ryuma, a legendary samurai from Wano whose corpse was reanimated by Gecko Moira on Thriller Bark with Brook's shadow. Following Ryuma's defeat, Zoro took his sword, Shusui — Zoro would later go on to return Shusui to Wano, trading it out for Kozuki Oden's cursed Haki-draining sword, Enma.
7 Nigiri: Hirameki
Another attack that utilizes the Nigiri stance, Hirameki is a quick-draw attack that sees both of Zoro's swords slashing to the right. As a testament to its name, which translates to "Flash", the technique is performed relatively quickly, acting as a reliable offensive response to oncoming enemy assaults.
The slashes also create two air-based projectiles, which, like many of Zoro's other techniques, are sent toward his target. And like most of the Nigiri techniques, its name is a play on hirameki being a type of sushi buffet.
6 Sai Kuru
Sai Kuru is another one of Zoro's Nitoryu-style techniques that effectively handles large crowds. When initiating this attack, Zoro holds his swords vertically in front of his body, then proceeds to spin in such a way that his blades create a whirling barrage of slashes that hit multiple opponents.
An example of this attacks' functionality is in Zoro's effortless elimination of the Franky Family, who attempted a surprise attack on Zoro. Much to their chagrin, Zoro easily incapacitated many of them at once, using Sai Kuru to handle them instantly.
5 Dai Gekken
While only seen once due to its situational usage, Dai Gekken is an attack that, when used, served tremendous damage against the Straw Hats' giant opponent, Oars. To increase his swords' power, Zoro combined them with Franky's heavy nunchaku, which were crafted out of two stone pillars.
With the increased weight and size (and support from being launched by Sanji), Zoro's slashes with his swords were able to deal remarkable damage to Oars. And while this attack has seemingly been a one-time use, it's undeniable that it was one of Zoro's strongest and most strategic.
4 Nanajuni Pound Ho
A technique with both offensive and defensive properties, Nanajuni Pound Ho is an improved version of an Ittoryu technique, Sanjuroku Pound Ho. In both techniques, Zoro uses air projectiles to deliver devastating blows, allowing him to make powerful attacks from a distance. As opposed to Sanjuroku Pound Ho, which uses only one projectile, Nanajuni Pound Ho sends two air projectiles at Zoro's target, doubling the damage output.
Along with the ability to deal damage, this technique can also be used to counter attacks. Zoro did this in his fight against Kaku (a fight that added to his bounty), in which he reflected one of Kaku's attacks using the air projectiles he created.
3 Nitoryu Iai: Rashomon
Nitoryu Iai: Rashomon is arguably one of Zoro's mightiest attacks given the feats he's been capable of achieving with it. Unlike many of the techniques on this list, Nitoryu Iai: Rashomon is a sword-drawing technique; Zoro draws two swords and sheathes them in a matter of moments.
In the time between drawing and sheating, Zoro was able to spit two train carriages in half, merely exemplifying the capabilities of this technique in that one instance. He would later use this technique against the powerful Devil Fruit user, Bartholomew Kuma. Although Zoro has yet to unleash this technique post-timeskip, one can only imagine how Zoro's increased strength would translate to this attack.
2 Nanahyakuniju Pound Ho
An improved version of Nanajuni Pound Ho, this version of the technique boasts an increased power boost due to Zoro's own power-levels increasing from his two years of training. The name itself demonstrates the technique's improvement, going from Nanajuni (72) to Nanahyakuniju (720). Like in its previous form, Zoro holds his two swords above his shoulder and delivers a destructive slash that forms two projectiles aimed at his target.
Although in its debut it was deflected by Kyoshiro, Zoro was confident enough that this ability could have instantly killed his opponent, Kurozumi Orochi.
1 Nigiri: Toro Samon
As a combination of two Nigiri-stance techniques, Nigiri: Toro Samon is an attack that optimizes Zoro's classic arsenal but improves upon them by merging them into one. As standalone techniques, Toro involved Zoro slashing two swords upwards with two air projectiles, while Samon saw two air-based projectiles sent diagonally downward.
Now used as a combined technique post-timeskip, Zoro has grown powerful enough to completely cut through opponents. This was seen when he sliced completely through Basil Hawkins, another rookie with one of the most powerful crews and highest bounties. This, in accordance to Zoro's newly acquired ability to generate and slash through flames, makes the gravity of this attack all the more powerful.