The former members of the Marine Admirals were:

Aokiji

Aokiji was the first Marine Admiral that was introduced in the series. After he lost his fight to Akainu he left the Marines and it is rumored that Aokiji then joined the Blackbeard Pirates. Aokiji saved the lives of thousands of Marines during the Marineford War. He possesses the Hie Hie devil fruit which allows him to create, control and transform ice at will.

Aokiji's haki is also known to be very strong. He is famously known to have saved and reduced the casualty that would have been caused during the Marineford War by a landslide.

Akainu

Akainu was promoted to Fleet Admiral after his battle with Aokiji. He believes in the concept of absolute justice and makes sure that justice is carried out against criminals at any cost. He possesses the Magu Magu devil fruit which gives him the ability to transform and manipulate magma. This gives him incredible attacking power. He was able to fight Whitebeard (the Strongest Man in the World) and even managed to scare away the Blackbeard Pirates.

Akainu is one of the most hated characters in the series as he is responsible for the killing of a lot of innocent people. He is famously known to have gone up against Whitebeard as not a lot of people would have the guts to go up against the Whitebeard Pirates.

Sengoku

Sengoku is known as the Fleet Admiral who held his ground during the Marineford War and led the Marines to victory. He was held in high regard by Gol D. Roger and also was able to capture Shiki. He possesses the Hito Hito devil fruit which gives him the ability to transform into a golden Buddha. In this mode, Sengoku can create shockwaves from the palms of his hands and he also possesses the conqueror's haki.

Sengoku is known to be the strongest character in the One Piece series. His power is so great that Gol D. Roger also acknowledged it which is considered a huge accomplishment.

Zephyr

Zephyr joined the Marines as a teen and with his abilities he rose up to the ranks rapidly and soon became a Marine Admiral. He was responsible for training many Marines who turned out to be heroes like Akainu, Kizaru etc. He did have a devil fruit but he was a great user of haki.

Even though he did not have a devil fruit like the other Marines, he was strong and only needed to depend on his haki. During his time as an Admiral he was in a league of his own.

The newly formed Marine Admirals are:

Kizaru

Kizaru is first seen during the Sabaody Archipelago arc where he was fighting against the Worst Generation. Kizaru possesses Pika Pika devil fruit which gives him the ability to create and manipulate light at will. He is known to be one of the strongest Marines in the series.

He is considered the strongest because of his devil fruit. His abilities make him the a very dangerous person and not a lot of power match up to his speed. His biggest achievement was attacking Law's ships.

Fujitora

Fujitora is seen for the first time during the Dressrosa Arc and he joined the Marines just after Ryokugyu. Fujitora had the Zushi Zushi devil fruit which allows him to control the gravity around him. Due to his devil fruit, he has the power to bring down powerful meteors and he can use them as weapons to destroy islands easily.

He has also fought with Sabo and didn't even break a sweat which saw Sabo acknowledged the fact that he couldn't find a way to get past him. He also fought with Jack from the Beast Pirates. Fujitora was known to take the blame for the incompetence of his subordinates during the Dressrosa Arc.

Ryokugyu

We see Ryokugyu for the first time during the Reverie arc and he joined the Marines after Aokiji and Akainu fought. He immediately joined as an Admiral and a lot of information about him is unknown. According to Doflamingo, both Fujitora and Ryokugyu are beasts in fights due to their monstrous strength.

A lot is not known about his abilities including which Devil fruit he possesses. It is known that he hasn't consumed any food for three years and despite that, he is able to function like a normal human being and can still fight at his full strength.

Q. Who is the strongest Marine Admiral?

A. Sengoku is the strongest Marine Admiral.

Q. Who is the weakest Marine Admiral?

A. Zephyr is the weakest Marine Admiral.