Who is the physically strongest villain, All for One or Muscular? In the battle with Izuku, Muscular was able to "cause destruction on a similar scale to Izuku's 100% Detroit Smash. He was also able to directly overpower Izuku, who was using One For All 100% at the time, in a physical clash." 
In the other hand, in the last battle of All for One with All Might, All For One says he is now confident of trading punches with All Might with his new strenght augmentation quirks, and he does indeed. 

Who is supposed to be the physically strongest villain, All for One or Muscular? Does the manga give more info about their strenghts?
I would like to start with this: In his fight with Muscular, Izuku was not using One For All with 100%. That was just Izuku with 100%. 

We saw the glimpse of All Might's power with his fight against first Nomu in season one. That Nomu was designed for him however All Might was still able to defeat Nomu.

Now if we think about those we can say that All Might could have defeat Muscular which is defeated by Izuku that can't even use OfA with its full extent.