The Marines are a major power in the One Piece world, known to be the military face of the World Government. As naval soldiers, it is their duty to protect the innocent and the weak from the vicious pirates, even more so in the Great Pirate Era ushered in by the Pirate King, Gol D. Roger. The Marines are quite a capable force and together with the SSG (formerly the Shichibukai) and the Yonko, make up the Three Great Powers of the One Piece world. Out of all the Marines, the Admirals are known to be the very strongest.
Each generation of the Navy seems to have maintained the tradition of having 3 Admirals and together, they're known to be the strongest military asset of the World Government. The Admirals possess tremendous power, as seen on various occasions in the story, however, whether they can compete against the Yonko is a different question altogether.
How Strong Are The Marine Admirals?
The three Admirals make up the biggest weapon that the World Government have at their disposal. On countless occasions, fans have seen how strong they truly are. During the Paramount War, the Admirals showed their might to the fans by taking on the mighty Whitebeard Pirates. While Aokiji clashed against Jozu, Kizaru took on Marco, and Akainu, the strongest of the three, took on Whitebeard himself. All the Admirals have overpowered Devil Fruits at their disposal and this is true for both before and after the timeskip. Before the timeskip, fans could see all three Admirals with overpowered Logia Devil Fruits, with Aokiji wielding the Hie Hie no Mi or the Ice Logia, Kizaru wielding the Pika Pika no Mi or the light Logia, and Akainu wielding the Magu Magu no Mi or the lava Logia.
After the timeskip, the pattern seems to have changed, however, their overpowered Devil Fruits still remain. Fujitora, the new Navy Admiral chosen with a World Military Draft, has the power of the Zushi Zushi no Mi or the gravity Paramecia. Meanwhile, little to nothing is known about Admiral Greenbull. When it comes to Haki, all the Admirals have shown the ability to use advanced forms of this power. During the Marineford arc, fans saw all three of the Admirals using Ryuo to deflect Whitebeard's quake, showing that, at the very least, they have the advanced form of Armament Haki. It wouldn't be a surprise to see the Admirals having advanced Observation Haki as well since they're incredibly monstrous fighters. With all those powers up their sleeve, it is no surprise that they're considered to be the absolute top-tier fighters in the world of One Piece.
The Admirals Vs The Yonko
While there is no denying that the Admirals are plenty powerful, comparing them to the Yonko is a different thing altogether. For one, the Yonko seem to be generally stronger and have a much better arsenal of abilities up their sleeve. For instance, during the Marineford arc, Akainu could fight Whitebeard for a while, however, it must be noted that Whitebeard's health was severely holding him back and he simply wasn't in fighting condition, to begin with.
Despite that, Whitebeard did eventually beat Akainu and that came at a point where he was far from his full power. Compared to a younger Whitebeard or, say, someone like Kaido, the Admirals aren't all that impressive just yet. After all, there is a reason why to fight the Whitebeard Pirates, the World Government absolutely needed the help of the Seven Warlords of the Sea. Simply using the Navy Admirals and the Vice Admirals against the Yonko wouldn't have been enough. If each Admiral was as strong as a Yonko, then the Navy would've had three Yonko level fighters compared to just the one for Whitebeard Pirates.
In contrast, however, Sengoku openly declared that despite having the numbers and even the Shichibukai at his side, they could very well be sunk to the bottom of the ocean by an old and dying Whitebeard and be utterly destroyed. Furthermore, commanders like Marco could hold their ground well against the likes of the Admirals until Whitebeard's worsening health distracted them and allowed the Admirals to take the upper hand.
That's certainly not to say that the Admirals are as strong as the Yonko Commanders. Clearly, Marco is an exceptional case, being the strongest of the commanders and someone worthy of becoming a Yonko himself in the eyes of the Gorosei. However, the Admirals aren't on the same level as the Yonko either.
To put that into perspective, Doflamingo had absolutely no problem fighting against Kuzan on Punk Hazard and even attacking Fujitora when he arrived on Dressrosa. At the same time, the mere thought of facing Kaido in battle and being a target of his wrath terrified him. Another example would be Shiki, who attacked Marineford all on his own and needed the combined effort of both Garp and Sengoku, who were both acknowledged by Roger, to be defeated after hours of battle.
Despite that, Shiki single-handedly destroyed half of Marineford. There is quite clearly a gap between the skill of the Admirals and the Yonko. Just the mere presence of Shanks being able to stop the entire Paramount War is yet another indication of that. Narratively, the Admirals have always been shown to be fighters who can take on the Yonko in combat and hold their own, but eventually, fall short. In terms of power, the Admirals, once again, don't compare to the Yonko, who not only have powerful Devil Fruits up their sleeve but also have the ability to use skills such as advanced Conqueror's Haki, one of the greatest skills in One Piece, if not the greatest.
Of course, the Admirals certainly aren't weak fighters. They're more than capable of beating the Yonko Commanders and even taking on and injuring the Yonko as well. However, from what the story has told the fans so far, they simply don't have enough power to take down the monstrous figures such as Kaido, Big Mom, Shanks, and the menace known as Blackbeard.