The Marines are one of the major power factions in the world of One Piece. They are the military face of the World Government and consist of some tremendous fighters, who are needed to curtail the Great Age of Piracy that is ongoing in the world of One Piece.
Within the Marines, there is a mix of powerful Devil Fruit as well as Haki users. Of course, every Marine with the rank of Vice Admiral and above is a user of Haki, however, the level of their Haki certainly depends on how much they have trained it. Not every user of Haki is as powerful as the others.
10 Smoker
Smoker is one of the Vice Admirals of the Navy and he is a skilled user of both Devil Fruit and Haki. Smoker possesses the Moku Moku No Mi, which is a powerful Logia type of Devil Fruit. His Haki is pretty good as well, as seen in the Punk Hazard arc.
Smoker was able to resist the powers of Law's ROOM to quite an extent. He could also put up a fight against Vergo, who was known to be a master of Armament Haki. While not the greatest, Smoker possesses very good Haki which has likely only grown over the course of time.
9 Momousagi
Momousagi is a Vice Admiral of the Navy who was once in contention for an Admiral spot. The fact that she was up for this position automatically makes her one of the very strongest Vice Admirals in the Navy. Fans do not know her to possess any Devil Fruit power as of yet, which means that the vast majority of her powers likely come from her powerful Haki.
Even if she does possess a Devil Fruit, for an Admiral, having a great Haki is the bare minimum, meaning her Haki is likely very strong.
8 Tsuru
Tsuru is a legendary figure in the world of One Piece who was famously part of a trio, with Garp and Sengoku. All three of them were known across the world and while Garp and Sengoku appeared to have been stronger, Tsuru was not weak at all. She is still considered to be a legendary figure within the Marines and that only means that her skill in combat was tremendous.
Being a Vice Admiral, she is certainly a user of Haki. Although her Haki has likely weakened due to old age, in her prime, it must have been tremendously strong. An indication of her level of Haki would be the fact that Doflamingo never wanted to face her in combat and ran at sensing her mere presence.
7 Ryokugyu
Ryokugyu is a Marine Admiral who was elected through the World Draft during the timeskip. He has a twisted sense of justice and tremendous power to put the justice into effect.
He is a user of the Mori Mori mo Mi, and on top of that, he is also a skilled user of Haki. Fans have not seen how powerful his Haki is, however, being an Admiral, it only makes sense for his Haki to be far above average.
6 Kizaru
Kizaru is one of the Three Admirals of the Navy as well and he has served them for quite a long time. He is a user of the Pika Pika no Mi, and his powers allow him to control create, and become light itself. He also has great Haki, as seen in Marineford.
He managed to stop one of Whitebeard's quakes with his Haki. He also appears to be a very skilled user of Observation Haki as he evaded most of Whitebeard's attacks effortlessly.
5 Fujitora
Fujitora is another Navy Admiral who was recently elected through the World Draft. He possesses tremendous powers of gravity, thanks to the Zushi Zushi no Mi. On top of that, he also has great control over Haki and he has already clashed with the likes of Luffy and Zoro and even the Chief of Staff of the Revolutionary Army, Sabo.
His Haki is very refined and the fact that he can even fight on par with all these incredibly powerful characters despite being blind just goes to show how great his control over Observation Haki is.
4 Kuzan
Kuzan is a former Marine Admiral and currently, he serves the role of the 10th Titanic Captain of the Blackbeard Pirates. Kuzan has the power of the Hie Hie no Mi, but he also has tremendously overpowered Haki. His Haki was put on display during the Marineford arc and, later on, he also used it extensively in his fight against Akainu.
During the war of Marineford, he evaded some pretty powerful Haki-infused blows with his Observation Haki and dealt great damage with his Armament Haki.
3 Sakazuki
Sakazuki is the current Fleet Admiral of the Navy and likely the strongest member of the Marines at the moment, aside from maybe Sengoku and Garp. He is extremely powerful and relies on his Devil Fruit quite a lot. However, at the same time, he has made sure that his Haki is tremendously overpowered as well.
Sakazuki possesses Haki which is great enough to stop some attacks from Whitebeard. He also appears to be a user of future sight Observation Haki. With advanced forms of both the Color of Observation and Armament, Sakazuki is a force to be reckoned with. Fans should also not be surprised if he is shown to wield the Color of the Supreme King in the future.
2 Sengoku
Sengoku is the former Marine Fleet Admiral and one of the only two members of the Navy to have full control over all three Haki types. Sengoku can wield the Color of Armament, Observation, and Supreme King. He also has a powerful Mythical Zoan Devil Fruit on top of this, which makes him unstoppable and combat.
In fact, his skill was such that even Roger considered him to be a worthy opponent to take on in combat. The fact that Sengoku wields the color of the Supreme King means that he's one of the very few people capable of using it in combat in the entire world of One Piece. This in itself means that his Haki is extremely overpowered.
1 Garp
Garp is the strongest marine to ever exist in One Piece. Fans already know that Garp has control over all three types of Haki. Fans know him to be a good user of Observation Haki. When it comes to Armament Haki, his power is second to none.
Of course, Garp is also a user of the Color of the Supreme King and he demonstrated its usage recently in One Piece when he fought against the Blackbeard Pirates on the Pirate Island. Being a rival of Roger, it only makes sense that Garp is a cut above the rest in the Marines when it comes to Haki.