Roronoa Zoro is one of the strongest people a piece And of course an incredible master of Haki. After learning the skill during the 2-year timeskip under Dracul Mihawk's tutelage, Zoro has impressed with his Haki several times. Of course, we know that Zoro is a user of all three types of Haki and he can use them very well. However, when it comes to the Haki attribute, Zoro's armor is the color, or Armament Haki. It is this art that he excels at the most and he has some great achievements using this form of hockey.

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Monet's Face of the Shepherd
Zoro's first exposure to Armament Haki came during the Punk Hazard arc, where the Straw Hat Pirates were up against the Logia type in Monet. We know that Monet was incredibly difficult to deal with to non-hockey users. This is where Zoro entered. Although he held back from taking her down due to her questionable principles in the fight against him, Zoro still tried to intimidate her by grazing her face with his sword.
This indicates that Zoro had strengthened the Haki in his blade enough to make the money bleed. However, in the end, Zoro didn't want to kill Monet and it just came down to the fact that he didn't want to cut the woman. It was Tashigi who finished the job and beat him.
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Overpowering and cutting full body Haki Pika
In the Dressrosa arc, we got a proper glimpse of Zoro using Armament Haki. This came during the fight against Pika, where Zoro was at the peak of his abilities. Armed with all three of his swords with Haki, Zoro was able to use their cutting edges to their full extent and cut through Pika's giant mountain-sized statue without even a flick of his blade.
If that wasn't enough, Zoro then went up against Pika, who was in his full Haki form like Virgo, and was able to cut him down and defeat him in the process. This remains one of Zoro's best Armament Haki feats in one piece.

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Haki was able to withdraw from Nma
In the Wano Country arc, Zoro finally gets a new blade named Enma, which he got after returning Shusui to his land. Zoro initially had some trouble dealing with the Enma, as it is a blade notorious for draining a person's Haki.
As such, the moment Zoro took control of Enma, it began to destroy his Haki. Still, Zoro was powerful enough to call Hawkeye back and take seconds later. Of course, it still took Zoro a long time to master this blade, however, the fact that he was able to command the blade to release Haki is impressive in itself.
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Cutting off Kaido's scales with Enma
Zoro went up against Kaido several times on the roof of Onigashima. There, he was able to bring out the best of Enma – a very powerful great grade sword. Even Kaido was wary of this sword because it was unlike any sword he had seen before. Due to his ability to drain Haki from a person's body, Zoro was able to cut through Kaido's scales with the high-grade Haki that was emanating from his body.
While not his greatest hockey feat to date, it was still an important one as hardly anyone would have cut him. The massive wound Zoro inflicted on Kaido only came as the sword began to siphon the victor's Haki from Zoro's body. Still, his Armament Haki also played a role here and Zoro certainly deserves credit for that.
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Protected against a point blank range explosion
In the Wano Country arc of One Piece, Zoro finally ends up fighting against King the Wildfire. It was a very difficult fight for him, and at one point in the middle of this fight, the king decided to blow himself up in Zoro's face. Zoro essentially died as the explosion was so large that it spread several meters around the Right Brain Tower on Onigashima. The fact that he was able to take a point-blank blast from the King shows that his Haki is very powerful.
Zoro then went to defeat the king. However, this armament was not derived from hockey, but rather from the awakening of victor's hockey. However, once again, it should be noted that his Armament Haki played a role in Zoro winning this battle, and it just goes to show that he is an excellent user of this ability.
One Piece manga chapters are available to read through Manga Plus.