Cavendish is a unique one-piece character that has multiple personalities when he falls asleep. Hakuba comes to life when Cavendish falls asleep. Hakuba is very fast by nature and I think this might be the reason to why he is very powerful. He was able to slice many strong people in Colosseum battle and was even able to beat Dellinger in a couple of seconds. Habuba seems to be another personality within cavendish or maybe its due to the sword that cavendish carries. I believe that cavendish has a cursed sword just like zoro and each cursed sword has different style and working. We saw zoro using his cursed sword for the first time against Kaku of CP9 just like that maybe Hakuba form is due to the cursed sword.
9. Killer
Killer is the Vice Captain and also a member of the worst generation. Just like Zoro he has a unique fighting style. He seems to be very strong just like Zoro and has a bounty of 200 million Belli. There are many theories regarding killer, his personality and his past as nothing related to him and his background has been revealed yet. I believe that Killer has some connection with Wano kuni as well and that is the reason why he will have some important role in Wano arc as Eustass kid is also now imprisoned inside the cell of Kaido. He has a unique sword style just like zoro's three sword style and maybe due to his unique amazing power he is also mentioned among the worst generation.
8. Vista
Vista also is known as Flower Blade Vista was the 5th Division Commander of WhiteBeard Pirates. The Status of Vista now is Still unknown but some hints to the style of swordsmanship has been given to us. He could bring petals of different flowers using his fighting style.Even during his Fight against Mihawk, he was acknowledged as a strong fighter. The last time vista and other remains of whitebeard pirates were mentioned was when they got involved in a war against blackbeard pirates that was also known as the payback war. The outcome of the war was that they lost and since then the status of vista is unknown.
7. Zoro
Roronoa Zoro is a fan favorite character who is the Vice Captain of Straw-hat Pirates having a bounty of 320 million Belli. He is currently the student of Worlds greatest swordsman Dracule Mihawk.He uses unique 3 sword style and is a very powerful character as well. Specially after 2 years time skip it seem like zoro is very powerful and is capable of using all three haki just like luffy. He was easily able to defeat one of Doflamingo's family member and was also able to fight one on one with Admiral Fujitora showing us how powerful he has become. Hence, i believe he is powerful swordsman.
6. Fujitora
Fujitora is a Navy Admiral that was introduced to us in the Dressrossa Arc. He is a Blind Swordsman. He is a very strong character who uses his devil fruit ability in his swordsmanship as well. He was able to stop Zoro very easily and since he is having a admiral status he must also have amazing haki. It was shown as if he has not even shown us half of his power in dressrossa. Even doflamingo was amazed to know that fujitora came to dressrossa. Doflamingo also called both the new navy admirals including fujitora and greenbull as beasts in power. Doflamingo seemed scared and amazed when fujitora showed him the glance of his power during the fight between fujitora, doflamingo and law.
5. Shiryu
'Shiryu of the rain' also known as Shiryu is a mysterious character who is apparently vice-captain of the Blackbeard Pirates. Before becoming the viceadmiral of blackbeard pirates he was the head jailer of impel down along with Magellan. It was mentioned that Shiryu and Magellan both were very powerful and were about same level in power. If Magellan single handedly took care of blackbeard pirates and nearly killed them in their first meeting we can just imagine how strong Shiryu might be when they both magellan and shiryu are on same power level The Power of Shiryu is yet unknown but he was said to possess great power that was equal to that of Magellan hence Shiryu is believed to be a very strong character.
4. Shanks
Shanks is one of the Yonko and is considered to be a very powerful OnePiece Character. He is a swordsman as well as Whitebeard even stated that Dracule Mihawk and Shanks used to get into many duels and no conclusion to those duels have yet to be revealed. After Shanks lost his left arm Mihawk lost interest in shanks. Even though Shanks does not have Left arm he is still a Yonko and was able to hold himself against kaido and whitebeard. We saw the sky getting divided and damage in whitebeard's ship when shanks used haki. He also stopped kaido during marineford fight leading me to believe that he is very strong.
3. Rayleigh
Silvers Rayleigh was the Vice Captain of the Roger Pirates and is considered to be a legendary pirate just like Whitebeard and Roger. He is also a Swordsman with excellent Haki skills. Even though not much of Rayleigh fighting has been shown he single-handedly stopped Kizaru. Marine Official such as Garp and Kizaru also said that he is a very tough person to fight. He was the person responsible in teaching luffy all forms of haki and he trained him for 2 whole years. Eventhough nothing much has been shown to us about rayleigh he seems to be an interesting character that will have important role in the future as well.
2. Gorosei
This Gorosei member whose name is not disclosed yet seems to be a very strong Swordsman. I believe that he was the strongest Swordsman Before Dracule Mihawk. He is always shown carrying a sword with himself. There are many type of swords inside onepiece storyline and i believe the sword that he is carrying always is a special sword just like Zoro and Mihawk's sword.
1. Dracult Mihawk(Hawkeye)
As the title suggests Dracule Mihawk is said to be the current Best Swordsman in the world. We have just been shown a glimpse of his powers nothing related to his history of the title and his full powers have been shown yet. Even though he is just a warlord it was told by whitebeard that he used to fight one on one with shanks. If mihawk could fight a yonko single handedly then he must be very powerful. He also taught zoro all forms of haki hence,he might also have strong haki.