According to an interview with Eiichiro Oda, he stated that if Akainu were to become the protagonist, he is so strong that he could end the One Piece series within a year.
Akainu possess an immense amount of superhuman durability (as not even Marco, Vista, and Jinbe, who are highly skilled Busoushoku Haki users, could not deal any significant damage to him) and a tremendous strength.
He ate the Magu Magu no Mi, a Logia-class Devil Fruit which grants him the ability to transform his body into magma and manipulate it at will.
Akainu’s ability is naturally of a higher order than Ace’s pyrokinetic abilities, as the higher heat and smothering abilities of his magma allowed him to successfully harm Ace despite Ace’s Logia-defense, and ultimately kill Ace due to this superiority.
During the decisive ten day battle on Punk Hazard for the position of fleet admiral, Akainu showed the furious power of his Devil Fruit as he changed the weather of one side of Punk Hazard and left it in a blaze.
It was stated that all vice admirals possess Haki, so being two ranks above vice admiral, Akainu can also use this ability. He was seen using his Busoshoku Haki to negate Whitebeard’s attempt to destroy the execution platform, by creating, with his fellow admirals, a shield combination of their three Haki.