>>the difference the light, the aperature, the iso, or something else
The difference is mainly the aperture. I was shooting wide open with a 1.4x extender attached. ISO 200, f/2.8, 1/3000, moving subject. Only the eyes and nose are in sharp focus. Yet (at full size) you can see tiny veins in his eyes and freckles on his nose that are pin sharp. You are comparing apples and oranges.
Sooooo hard to pick a favorite. There are 4 or five I could put up here. I love all of my lenses, but since my first photography love was and still is Wildlife, I guess it has to be the 400mm f/5.6L
I don't have one either.