Well, it's a bit hard to say for sure. But some fans believe Benimaru Shinmon could be the strongest, given his immense power and combat skills. Different characters shine in different situations.
In the Mortal Immortal World Chapter, the strongest power in the Immortal World was the Nine Elements Temple.
Jiuyuan Temple was Samsara Temple's mortal enemy. It formed the Heavenly Law Alliance with Truth Sect, Candle Dragon Sect, Exquisite Sound Sect, Qingyuan Palace and other forces to resist Samsara Temple. The sect master of the Nine Elements Temple was one of the top mighty figures in the Immortal World. His strength was unfathomable. The cultivation techniques and magic treasures of Jiuyuan Temple were also very powerful. They were treasures that immortal cultivators in the immortal world dreamed of.
While waiting for the TV series, he could also click on the link below to read the classic original work of " The Legend of Mortal Cultivation "!
Shinra is definitely one of the most popular. His unique power of the 'Devil's Footprints' and his determination to find answers about his family and the mystery behind the Infernals make him a great focus. Also, Arthur with his delusions of being a knight but also having real strength is a fan favorite. His comical yet brave nature often gets explored in fanfiction. Another popular one is Maki, especially for stories that focus on female empowerment within the Fire Force universe. Her strong will and powerful combat abilities make her an interesting character to write about.
Shinra Kusakabe is quite interesting. His ability to use his feet as jets gives him a unique fighting style. Then there's Arthur Boyle, with his delusions of being a knight, which adds a touch of humor to the story while also making him a formidable fighter. Also, Captain Obi is interesting as he leads the 8th Company and has to deal with all the eccentric members under his command.