It could be the main protagonist if the story is written in a way that the character manages to control and utilize all the Akatsuki powers effectively. With the combined powers, they would have a vast array of abilities at their disposal, making them extremely formidable.
There might be an original character created in the fanfiction who is designed to be the ultimate wielder of all Akatsuki powers. This character could be a long - lost relative of the Akatsuki members or someone with a special connection to their powers. They would have trained specifically to handle the combined powers, and thus be more powerful than anyone else in the story. Their power would be a blend of the diverse Akatsuki abilities, like being able to summon the powerful beasts that some Akatsuki members are associated with, while also having the ability to use the unique ninjutsu of others.
Often, in these types of fanfictions, an antagonist who steals or is given all the Akatsuki powers becomes the most powerful. They don't have the moral constraints that a protagonist might have, and can use the powers in more ruthless ways. For example, they could use Pain's ability to rain down destruction on villages without hesitation, combined with Itachi's mind - controlling illusions to subdue any opposition.
Well, it could be that there's a plot device like an ancient artifact. This artifact might have the power to draw out and combine the Akatsuki powers. The Akatsuki members themselves might be tricked into contributing their powers to this artifact, and then the protagonist or an antagonist gets hold of it and gains all those combined powers. It's a common trope in fanfiction to use such magical or mysterious items to bring about extraordinary power combinations.
Konan is often very popular. Her paper - like abilities are really cool and in fanfiction, she can be portrayed in many different ways, like a more sympathetic figure who is just caught in a bad situation.
If you combined Naruto's determination and power - up abilities with the Otsutsuki's unique dojutsu and chakra - related powers, and then added the Akatsuki's knowledge of various jutsu and their strategic thinking, it would be an extremely powerful character. Naruto already has the Nine - Tails' chakra and his Sage Mode. The Otsutsuki have things like the Rinnegan. And the Akatsuki members are no slouches in combat either. So this combined character would be able to take on just about any foe.
Pain would be one of the most challenging. His Rinnegan gives him a wide range of powerful abilities like the Six Paths of Pain. Naruto would have to figure out how to counter each of those paths and find a way to reach the real Pain. Itachi is also tough. His Sharingan and genjutsu skills are top - notch. Naruto would need to break free from his genjutsu and then find a way to fight back against his fire - based attacks.
One of the most popular female characters in Akatsuki in fanfictions is Konan. Her unique paper - based abilities and her complex relationship with Nagato and Yahiko make her a fascinating subject. In many fanfictions, her backstory is explored more deeply, especially her time growing up in the war - torn land of Amegakure. Another popular one could be Karin. Her connection to Sasuke and her own powers as a sensor type ninja give a lot of room for fanfiction writers to create interesting storylines, like her dealing with her feelings for Sasuke while being involved with the Akatsuki.
Pain is also popular. His powerful abilities and his ideology about peace through pain are fascinating. In a fanfiction where Naruto is in the Akatsuki, Pain's reaction to Naruto's presence and how they interact would be a big draw for readers. Kisame might also be popular. His loyalty to the Akatsuki and his shark - like appearance and abilities add an interesting element to the story.
Naruto is obviously very popular as he is the main character of the original series. His interactions with the Akatsuki members are always a focal point in fanfictions.
Itachi is often a very popular character in Akatsuki fanfiction. His complex backstory, his powerful abilities, and his self - sacrificing nature make him a fascinating character to write about. Writers often explore his relationship with Sasuke more deeply or create scenarios where he didn't have to die.
Kakuzu is another character that can be popular in these fanfictions. His unique ability to have multiple hearts and his money - loving nature can add a touch of humor or be explored in more serious ways in relation to the Akatsuki's goals and activities.
Definitely Itachi. He has a complex character and his own sense of justice. He might see Naruto as a key to changing the shinobi world in a different way than what the Akatsuki originally planned. Another one could be Kisame. He is loyal to Itachi and would likely follow Itachi's lead in taking care of Naruto.