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.
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.
Chapter 48, "All the Bitter Teas Have Been Taken Away", is the clearest early answer because it turns Bitter Teas Have's situation into a concrete plot event. Early on, He Yun felt guilty for initially ignoring Jiang Li on purpose, and....
Chapter 114, "I've never seen someone so shameless!", is the clearest early answer because it turns Jiang Yan's situation into a concrete plot event. The chapter gives readers a real answer by opening on As time passed, the number of viewers in the live broadcast gradually...
Chapter 240, "He's the Last One?!", is the clearest early answer because it turns Jiang Yan's situation into a concrete plot event. The chapter gives readers a real answer by opening on Wuuuuh He Bao truly deserves it, Mom always knew you could do it!"...
Chapter 127, "He's Not Snobbish, Just Simply Bad-Tempered", is the clearest early answer because it turns Jiang Yan's situation into a concrete plot event. The chapter turns that into something concrete by moving from Witnessing Jiang Li leave with a bucket, Jiang Yan refused to admit...
Chapter 319, "Sister Xiao Wan", is the clearest early answer because it turns Jiang Cheng's situation into a concrete plot event. It opens with Staring at the screen displaying "Sister Xiao Wan," Jiang Cheng... and closes on The fate of key characters in the book will be affected, so you must....
Chapter 33, "Variety Show of Villains", is the clearest early answer because it turns Jiang Li's situation into a concrete plot event. A useful checkpoint is the way the scene starts with Following the announcement, five artists respectively reposted the...
Chapter 31, "Master, I've been having some bad luck lately!", is the clearest early answer because it turns Su Yinwan's situation into a concrete plot event. The chapter turns that into something concrete by moving from Before Shen Lan had reached the backyard, she already heard the...
Chapter 286, "I Won't Let Anything Happen to Either of Us", is the clearest early answer because it turns Primordial Forest's situation into a concrete plot event. It opens with Throughout these reincarnations, Zhao Lang had gradually discerned a...
Chapter 95, "The Problem Lies with Your Boyfriend", is the clearest early answer because it turns Jiang Li's situation into a concrete plot event. It opens with Hearing others praise her daughter, Aunt Lin was smiling so wide she...
Chapter 264, "The Truth", is the clearest early answer because the hidden truth stops feeling like mere subtext.