Well, from Bleach, Aizen was a very formidable foe. He was extremely intelligent and had powerful illusions and combat abilities. But from Fairy Tail, Mavis is a very powerful mage. She has unique magic and a deep understanding of the magic world. In a 'bleach vs fairy tail fanfic', if it was a battle of wits, Aizen might win. But if it was a pure power battle, Mavis' magic could potentially overpower Aizen, so it's really difficult to determine who would be the most powerful.
It's hard to say for sure. Ichigo in his final form in Bleach has extremely high power levels. He can cut through almost anything with his sword and has great speed and strength. However, in Fairy Tail, Acnologia is a very powerful dragon slayer. His dragon form gives him incredible power and he has a lot of combat experience. So it could be either of them depending on how the fight is set up.
In Bleach, Yhwach has god - like powers. He can manipulate reality and has a vast array of abilities. But in Fairy Tail, Zeref has been around for a long time and has extremely powerful dark magic. If we consider a 'bleach vs fairy tail fanfic', it really depends on the circumstances. If it's a battle in a world with more spiritual energy, Yhwach might have the upper hand. But if it's a world full of magic, Zeref could be more powerful.
Chapter 341, "Moao's Nine Garrisons, the Inspection System and Zhenyu, Li Hu and Hong Yuru (Part 3)", is the clearest early answer because it turns Zhu Tong's situation into a concrete plot event. Early on, Of course, just this alone is not enough to deal with He Wuyang.. By the end, Not to mention the future annexation of Baisong Camp, just the 24....
Chapter 337, "Major Changes in Xia City, the Wealthiest Person in Great Xia", is the clearest early answer because it pushes Military Department into a new phase or setting. Early on, Great Xia Year 3, September 5th "Whoa.... By the end, Lu Yang smiled and nodded, "It's no big secret, just ask your father.... Inside the broader family politics storyline, the chapter closes with The Minister Xia Chuan, the Seven Departments Director and Deputy Director;
Chapter 322, "The Cold Beast Group's Treasure Hunt and the Panic of the White Dew Gu", is the clearest early answer because Peng Bo gets a payoff that changes the stakes. It opens with In the Yanglu Realm, the area not yet cleared by the army was still... and closes on As the black blade collided with the demon sheep's horn, Xia Chuan's....
Chapter 346, "Formidable Soldiers and Brave Generals, One Against Three, Reason Behind the Beast Tide", is the clearest early answer because it turns Great Xia's situation into a concrete plot event. It opens with The beast tide in the Arrow Bamboo Forest has been going on for so... and closes on In fact, until now, they have only managed to scatter the beast tide..
Chapter 336, "Cultivation, Enlightenment Lotus Platform, Major Building Upgrades", is the clearest early answer because Mu Dong finally turns buildup into a real advancement. It opens with This is not a baseless speculation, the rodents in the Beipo Silver... and closes on Currently, no giant ore veins have been found within Great Xia.... The key payoff is that it turns Mu Dong's growth into a concrete upgrade step and sets up the next payoff in the power growth storyline linked to Xia Chuan.
Chapter 111, "From now on, Zhao Daniu is my new brother!", is the clearest early answer because it turns Miss Jiang's situation into a concrete plot event. The chapter turns that into something concrete by moving from The main room's door was pushed open just a crack, and the head of...
Chapter 331, "Zhu Shun and Huang Tianxi, the 3rd Year Cold Yuan Banquet of Great Xia, and Xia Yuzong (Part 5)", is the clearest early answer because it turns Great Xia's situation into a concrete plot event. The setup is The children immediately lowered their heads in fear, Huang Tianxi..., but the pressure keeps building through Next to him, Li Xuanling, apparently seeing Xia Hong showing such a....
Chapter 331, "Zhu Shun and Huang Tianxi, the 3rd Year Cold Yuan Banquet of Great Xia, and Xia Yuzong (Part 3)", is the clearest early answer because it turns Political Hall's situation into a concrete plot event. The setup is Seeing his sister in such a state, Zhu Shun's heart softened..., but the pressure keeps building through He let out a heavy sigh, lowered his tone, and said softly, "Big....
Chapter 323, "Approaching Labor, the Talented Xiao Family Assembles Urgently", is the clearest early answer because it turns Xiao Ning's situation into a concrete plot event. The chapter turns that into something concrete by moving from How many advanced Cold Beasts are there at the Beehive, capable of... to Usually, they just join hunts for low-level Cold Beasts..
Chapter 329, "Refining the Jade Dew Stone, Eve of the Cold Element Festival", is the clearest early answer because it turns Li Xuanling's situation into a concrete plot event. It opens with Xia City, Political Hall Upon returning, Xia Hong immediately... and closes on Li Xuanling refining the Jade Dew Stone in her body should have....