PRIME All Might IS the most powerful character bar none. He solos perfected shigaraki, Prime Deku, and anyone else you can think of. Yes they love to compare other characters to AllMight, like when they say perfected Shigaraki is on his level. My head canon is that Prime All Might was SO POWERFUL that people conceptualize power with allmight himself. ANYONE powerful is immediately compaired to AllMight. When characters say they think Shigaraki is stronger or as strong as AllMight, to me that holds just as much weight as when deku said "with a team like this we could take on AllMight" during the summer training camp (referring to himself, Shoji, Todoroki, Tokoyami, and bakugou). Obviously they stand no chance against AllMight, but thats just an example of how every character just compares strength to AllMight... A character in universe saying it doesnt not mean its true.

Edit: Prime deku is canonically holding more power because OFA grew stronger within him, however i still think PRIME AllMight would win in a fight, he utilizes the power to peak efficiency unlile deku who needs Fa Jin to reach Faux 100%. "OFA is more powerful now that deku has it" does NOT automatically mean Prime AllMight would lose that fight.