Tell me what makes Aizen so unlikable as a villain. I'm sure you'll typically say "HURR DURR AIZEN IS OVERPOWERED AND SUPER DUPER SMART AND STUFF"
He's not just unlikeable as a villain - he's unlikeable as a character. His presence removes all suspense - every single time that there is any kind of cliffhanger which appears to have him injured in any way, we know that by the next chapter, he'll either be completely unharmed, or the entire thing will be an illusion. He has no personality beyond smug preparedness, and given that he recently made four years of the story completely pointless by defeating the Espada on his own and complete cellular destruction can't put him down, he quickly becomes frustrating to read about. He's merely a shell of a character, a male version of a Mary Sue, and given that a larger version of an attack that's already failed multiple times won't stop him, he will continue to be for the foreseeable future.
Also, we're still waiting on your list of reasons why Bleach is better than One Piece so that they can be completely proven wrong.