The fanbase loves this debate but honestly, I think a lot of people overlook how the series frames 'strongest' beyond raw destructive power. All Might at his peak is the obvious symbolic answer, but the narrative consistently shows us that Shigaraki's completed decay, with the speed and range he gains later, is a contender for the most overwhelming threat. It’s a force that can’t really be fought head-on by most characters. That said, if we're talking about a character who could win a straight fight against the broadest range of top-tiers, Star and Stripe's New Order was conceptually broken. The series had to write her out quickly because her power’s potential was narrative kryptonite.
My personal dark horse is Endeavor. He never gets the 'strongest' title, but his post-All Might journey is about redefining strength as relentless, honed force and brutal adaptability. Against high-end Nomu or other heroes/villains, his concentrated firepower and sheer will give him a shot where more straightforward powers might fail. He’s the grinding warrior archetype in a world of gifted prodigies and cosmic-level quirks, and that’s a different kind of strength the series respects.
Ultimately, I lean towards Shigaraki as the current in-story apex, but the fun is in the matchups. Could Deku’s perfected Fa Jin and Gearshift overwhelm decay’s activation? It’s the kind of hypothetical that keeps forums alive.