The “perfection” of a protein is measured in terms of how much of that protein is used by the body once it’s consumed. The world is full of different kinds of proteins after all, and not all of them break down the same way in the digestive tract. Some break down very inefficiently, meaning that much of the nutritional value that the protein carries (specifically its nitrogen) isn’t absorbed. The most “perfect” proteins are those that are the most “bioavailable”, i.e. give up the most of their nutrients as they break down. Egg proteins are 97% bioavailable, which is, you know, a lot.