Impel Down is one of the three biggest institutions that fall under the control of the World Government. It is a maximum-security prison created by them where they imprison the worst criminals in the One Piece world. Impel Down prides itself on being an impenetrable fortress that no criminal can escape from.
Since its establishment, the facility has contained all kinds of criminals, ranging from low-class to incredibly high-profile ones. The most dangerous One Piece characters are locked on Level 6. With so many threatening individuals locked in Impel Down, there is an obvious difference between their power levels.
9 Shiki
Shiki was a member of the Rocks Pirates, but he left after the God Valley Incident. He eventually started his own crew and gained a solid reputation in a short period of time. Shiki built an armada and challenged Roger. However, he lost the fight. When he discovered that Roger had turned himself in, he decided to invade Marineford.
Shiki fought Sengoku and Garp all by himself. It was revealed that Shiki managed to destroy nearly half of Marineford and critically injure Garp. Shiki was defeated by the two Marines and placed in Impel Down. But the prison couldn't hold him for long. Shiki managed to escape after chopping off his legs. Given his feats, he is easily the strongest pirate to ever step foot inside Impel Down.
8 Portgas D. Ace
Ace was the Second Division Commander of the Whitebeard Pirates. He ate the Mera Mera no Mi, a Logia-type Devil Fruit that powered his attacks. Ace was also among the few characters who could use all three types of Haki.
Ace ended up in Impel Down because of Blackbeard. The former Whitebeard pirate wanted to capture Luffy and hand him over to the World Government in exchange for a spot in the Shichibukai. Ace managed to find Blackbeard before the latter could harm his younger brother. The fight between the two pirates was intense, but Blackbeard managed to take down his former commander. Ace was then taken to Impel Down, where he was kept on Level 6.
7 Crocodile
Crocodile is a former Shichibukai and one of the two leaders of the Cross Guild. He was introduced in the Alabasta arc. Crocodile's plan was to take over the country with minimal effort, and he would have been successful in his mission if it weren't for Luffy's timely intervention.
Following his defeat, Crocodile was locked in Impel Down, where he met Luffy yet again. The two struck a deal and escaped from the facility together. Crocodile's Suna Suna no Mi made him a valuable ally, and it enabled him to save Ace from being executed.
6 Donquixote Doflamingo
Donquixote Doflamingo is the personification of evil. Everything that he did was motivated by his desire to be at the top. To him, it didn't matter what he had to sacrifice. Doflamingo ate the Ito Ito no Mi, which was given to him by Trebol. He used its power to secure a place in the Shichibukai and take over the country of Dressrosa.
Like many Shichibukai before him, Doflamingo's downfall was caused by Luffy. After being defeated, Doflamingo was transferred to Impel Down. He is locked on the lowest level, with Magellan trying to protect him.
5 Ivankov
Ivankov is a member of the Revolutionary Army. He is the leader of the G Army, which consists of all the Grand Line members. Considering the other commanders of the Revolutionary Army are able to fight against Admirals, it is safe to assume that Ivankov can do something similar.
Ivankov was locked on Level 5, but he managed to escape from his cell. He eventually created Level 5.5, which came to be known as the New Kama Land. Ivankov helped Luffy a lot during the Impel Down arc, and needless to say, the young pirate would have died without him.
4 Jimbei
Jimbei has been a part of two of the most popular pirate crews in the series. He started his career with the Sun Pirates, and he joined the Straw Hat Pirates in Wano. Before he became an official Straw Hat, Jimbei captained the Sun Pirates. He was offered a place among the Shichibukai by the World Government, and he accepted in order to protect Fish-Man Island.
He was removed from his position because he refused to fight against Whitebeard. So, the Marines locked him inside Impel Down as a punishment. Jimbei was freed during the riot in Impel Down, where he eventually joined Luffy's cause to save Ace.
3 Avalo Pizzaro
Avalo Pizzaro is the captain of the fifth ship of the Blackbeard Pirates. He was locked inside Impel Down for the many heinous crimes that he had committed, and as a punishment, he was put on Level 6. Obviously, the World Government wanted to erase his name forever.
Prior to his escape, Pizzaro didn't seem to have any Devil Fruit, but it is possible that he did have knowledge of Haki. Since he killed several other Level 6 prisoners, it is plausible to assume that he is a very strong individual.
2 San Juan Wolf
San Juan Wolf is a ginormous man who was imprisoned many years before the story. He is a giant, so he naturally towers above most of the characters in the series. San Juan Wolf lineage gives him incredible physical strength.
He ate the Deka Deka no Mi, which is a Paramecia-type Devil Fruit, at some point in his life. The Devil Fruit enables him to increase his size to the point where he could stand in the sea.
1 Shiryu
Shiryu used to be the head jailer of Impel Down, but he was removed from his position due to his psychopathic tendency to kill prisoners. Shiryu was locked away by Magellan on Level 6, and it wasn't until the riot caused by Blackbeard and Luffy that the head jailer was allowed to roam free.
Shiryu was considered to be on the same level as Magellan, and what's impressive is that he didn't even have a Devil Fruit at the time. Currently, he is a member of the Blackbeard Pirates.