Who is the physically strongest villain, All for One or Muscular?
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.