Out of the notable manga that I have read, the worst main character for me is Ichigo.
The funny thing with Ichigo is that I don't particularly find him irritating as a character. The fact that he doesn't have a gimmicky catchphrase that he yells every three chapters is a bit of an improvement over some other certain MCs.
Nah, what absolutely kills Ichigo as a main character for me is the writing surrounding him, particularly two main things about the writing:
I) Cause the story follows Ichigo wherever he goes and Ichigo lives in the utterly mundane, mind numbingly boring human world, Kubo drags the reader with him in order to watch agonisingly dull shite like high school.
Say what you will about the likes of Luffy or Naruto but at least through following these characters, we get to see the wider world that they inhabit due to the adventures they have. Not Ichigo though. Following Ichigo is an active hindrance to exploring the world of Bleach because Ichigo doesn't actually live in the interesting part of the Bleach world (the Soul Society), he lives in the stupidly boring normal world and only goes to the Soul Society when some crisis is in full swing.
Ichigo is an outsider that only briefly interacts with the Soul Society, so by following Ichigo we only get an outsider's brief view of the Soul Society. Lel.
II) Ichigo is supposed to be the protagonist but he doesn't have a single proactive bone in his body that moves the narrative of the story forward. Everything Ichigo does is a reaction to an antagonist doing something because the antagonists are the only people who actually get the plot moving in Bleach.
Ichigo is a main character with no drive, no goals, no aspirations and ultimately, no actual story. Following Ichigo isn't going on a journey, it is going on a random series of events that coincidentally have Ichigo involved in them.
Let me put it into perspective.
One Piece is a story about the journey of a young man who seeks to become the Pirate King and his search for freedom in a world of oppression, corruption and totalitarian justice.
Naruto is a story about the journey of a boy who seeks to become Hokage in order to gain the admiration and respect of those who have shunned him.
My Hero Academia follows the journey of a boy who seeks to become a hero in order to emulate his idol and save others while carrying the weight of a legacy.
Bleach is about... er.... a guy who gets spiritual superpowers and beats up whatever sassenach of the week is causing a general inconvenience, I guess.
Seriously, there is no journey, no story, there is nothing that actually connects anything in Bleach together apart from the fact that Ichigo is involved.
The plot treats Ichigo like he is a dog on a leash. It forcibly leads him to wherever it needs him to go and it is never moved forward by Ichigo himself.
If Byakuya and Renji had not come to capture Rukia then Ichigo would have been content to flex on low level Hollows within Karakura Town for the rest of his life. Compare that to Luffy, who no matter what will always aim to be the Pirate King or Naruto, who will always strive to be the Hokage, Ichigo simply has nothing to move him forward.
Ichigo is an utterly flaccid MC that is an active hindrance to the story. Hell, since Aizen is the one who is responsible for nearly everything that happens in this damn story, you could consider him a villainous protagonist and Ichigo a heroic antagonist. Aizen did far more to move the plot forward than Ichigo ever did.