Sakazuki, also known as Akainu, was introduced before the two-year timeskip of One Piece as one of the Three Admirals of the Navy and, thus, their strongest military asset at the time. Having been an Admiral, Akainu is one of the most powerful people in the world of One Piece. In the Enies Lobby arc, fans got their first glimpse of Akainu's terrible power when he blew up the ship carrying the people of Ohara. Shortly afterwards, Akainu was one of the stars of the Marineford arc where he challenged the likes of Whitebeard and his pirate crew, and made quite a name for himself.
During the two-year timeskip, he rose to the position of Fleet Admiral of the Navy and, unlike any other Fleet Admiral before, he is openly challenging the supremacy of the Yonko in the New World. The debate of whether he is strong enough to compete with the strongest people in the One Piece world has always existed but does Akainu really have the power to stand next to the legends?
Akainu's Devil Fruit
All the Admirals before the two-year time skip in One Piece had Logia Devil Fruits, with Akainu having the strongest of the lot at his disposal. While Aokiji was seen with the Hie Hie no Mi, which made him an ice human, Kizaru displayed the Pika Pika no Mi's light powers. Akainu, the strongest of the lot, demonstrated the incredible powers of the Magu Magu no Mi during the Marineford arc. With the powers of this Devil Fruit, Akainu can turn into magma at will, making him a magma man. Its destructive powers are in the same league as the likes of the Gura Gura no Mi of Whitebeard, one of the former Four Emperors of the Sea.
Furthermore, Eiichiro Oda has already confirmed that Akainu's Devil Fruit is one with incredibly high offensive power and belongs to the same category as the Gura Gura no Mi in its offensive output. As such, in terms of Devil Fruit powers, Akainu is quite easily one of the strongest in the entire world and arguably even the most destructive.
Akainu's Haki
While Devil Fruits are certainly quite important when it comes to combat in One Piece, they aren't what define the battles all the time. Along with a powerful Devil Fruit, a mighty strong Haki is just as important. According to Kaido, Devil Fruits are certainly great but Haki is what truly conquers all powers, and to back this up, he quoted the example of Gol D. Roger. In terms of Haki, Akainu is certainly quite impressive. He has already been confirmed to be a user of Observation and Armament Haki. During the Marineford arc, Akainu could survive some mighty powerful blows from Whitebeard thanks to his incredibly tough Haki.
Furthermore, he even demonstrated the skill to use Ryuo, which is an advanced Haki form, showing that he has indeed mastered the Color of Arms. What's even more interesting is the fact that he was able to evade the blows of Marco and Vista with Observation Haki, similar to how Katakuri does it, hinting at him possessing even Future Sight.
While Conqueror's Haki is something that he hasn't been confirmed to possess just yet, it could be a power that he very well has up his sleeve as well. The former Fleet Admiral, Sengoku, was a user of Conqueror's Haki. Furthermore, Akainu is openly challenging the supremacy of the Yonko in the New World and competing for power. As such, he is certainly someone with great ambition and he could very well have Conqueror's Haki.
Akainu's Portrayal
In terms of portrayal, Akainu is certainly among the most overpowered fighters in the One Piece world. Often, he is seen competing with the likes of the Yonko for supremacy in the New World. He has sworn to destroy every single one of them on several occasions and, most recently, after the conclusion of Wano, he once again showed that he was not backing away from a fight. What's more, Akainu is also a rival of Luffy. After all, he ended up killing Ace, Luffy's brother, and even scarred Luffy's chest. Being one of Luffy's biggest enemies, it is expected for him to be strong enough to pose a challenge to him.
Since Luffy is already a Yonko right now, Akainu would have to be somewhere close to him in power to pose a threat. As such, fans should expect Akainu to be close to the level of the Four Emperors. At the same time, it is important to note that the strength of the enemies increases the later they are shown in the series. Akainu being one of the final antagonists of the story, and one of the biggest ones, certainly puts beyond any doubt that he will turn out to be one of the strongest characters in One Piece.