All Might's AP/Dura is over 3x stronger than Ryoh, and his strongest attack wouldn't be enough to one shot, although it'll inflict severe damage.
However, Ryoh is far superior in terms of mobility. He can avoid All Might's straight forward attacks while landing attacks of his own. All Might doesn't dodge and just charges through everything. This isn't good since while Ryoh's AP might be lower, his LS is vastly higher.
All Might will be sent flying back with a single attack. If Ryoh desired it, All Might won't be able to get near him. All Might hasn't been shown to be proficient in long rang combat either. Meaning Ryoh should have the advantage in range, but there may come a point where All Might unleashes massive AOE air blast that cover the entire arena. Not saying All Might is range spamming, that's out of character. But he'll start unleashing massive AOE attacks, even up close, if he can't hit his opponent.
While a 3x advantage is good, it's not enough to one shot Ryoh, and his accelerated development will kick in at this point. As all of his stats increase, he'll become faster than All Might. Could he close the gap between their AP/Dura? I imagine this is when Ryoh would use his strongest attack.
Considering his accelerated development, is it possible the attack could put All Might down if he grew strong enough before using it? All Might himself will start going Plus Ultra as well. Comparing other Plus Ultra, he can become strong enough to stomp people on par with him.
I think this depends on if Ryoh can put All Might down with his biggest attack. All Might's stamina allowed him to keep fighting even after being disemboweled. If Ryoh's AD can put him on All Might's level, the 10x amp would be enough to one shot him. But if he doesn't or can't, I think All Might will take the win afterward.
I have a bias so I want to lean towards All Might, but I want to hear another opinion first.