Roronoa Zoro is one of the strongest individuals in One Piece and truly an incredible master of Haki. After learning the skill during the 2-year time jump under the tutelage of Dracule Mihawk, Zoro has continued to impress with his Haki on several occasions. Of course we know that Zoro is a user of all three types of Haki and that he can use them very well. But when it comes to Haki specialty is Zoro's Color of Arms, or Armament Haki. It is this art that he excels at the most and he has some of the best feats using this type of Haki.

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Grazed Monet's face
One of Zoro's first displays of Armament Haki came during the Punk Hazard Arc, where the Straw Hat Pirates encountered a Logia-type in Monet. We know that Monet was incredibly difficult for non-Haki users to handle. This is where Zoro stepped in. Although he avoided taking her down due to his questionable principles in his fight against her, Zoro still tried to intimidate her by grazing her face with his sword.
This implies that Zoro had enough strong Haki imbued in his blade that caused money to bleed. In the end, however, Zoro chose not to kill Monet and this was simply because he didn't want to cut a woman. It was Tashigi who finished the job and beat her.
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Overwhelming and cutting full body Haki Pica
In the Dressrosa Arc, we got a proper glimpse of Zoro using Armament Haki. This came during his fight against Pica, where Zoro was at the peak of his abilities. By infusing all three of his swords with Haki, Zoro was able to utilize their tip to the highest level and carve up Pica's gigantic mountain-sized statue without even a crack on his blade.
If that wasn't enough, Zoro then went up against Pica, who was in his full Haki form just like Vergo, and was able to cut through him and knock him out in the process. This remains one of Zoro's finest Armament Haki feats in One Piece.

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Was able to take Haki back from Enma
In the Wano Country Arc, Zoro finally got a new blade, called Enma, which he got after he returned Shusui to the country where it belonged. Zoro had a little trouble handling the Enma at first, as it is a blade that is notorious for draining the Haki of the individual.
As such, the moment Zoro used the Enma, it began to drain his Haki. Still, Zoro was powerful enough to claim Haki back and take it seconds later. Of course, it still took some time for Zoro to master this blade, but the fact that he was able to command the blade to release Haki is impressive in itself.
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Cut Kaido's wave with Enma
Zoro went up against Kaido on several occasions 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 as it was unlike any sword he had seen before. Due to his ability to drain Haki from an individual's body, Zoro was able to cut through Kaido's scales with the high quality Haki it drained from his body.
While it wasn't his best Haki performance to date, it was still a significant achievement as hardly anyone had ever cut him down. The larger scar that Zoro gave Kaido only came after the sword began to drain the Conqueror'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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Guarded against an explosion in an empty area
In the Wano Country Arc of One Piece, Zoro eventually ended up fighting King the Wildfire. This was a very difficult fight for him to get through, and in the middle of this clash at one point, King decided to blow himself up right into Zoro's face. Zoro would have basically died as this explosion was absolutely massive and spanned several meters around Onigashima's right brain tower. The fact that he could take a straight blast from King just shows that his Haki is very powerful.
Zoro then proceeded to defeat King. However, this was achieved through his awakening of Conqueror's Haki and not Armament Haki. Still, once again, it must be noted that his Armament haki played a role in Zoro winning this battle, and it just goes to show that he is an amazing user of this ability.
One Piece manga chapters are available to read via Manga Plus.