The opposite of "most undifferentiated" is "highly differentiated." Most undifferentiated means something that is not distinct or distinguishable, while highly differentiated means something that has many distinct parts or characteristics. For example, a highly differentiated company would have many departments and areas of expertise, while a most undifferentiated company would have few or no distinct areas of focus. Similarly, in biology, highly differentiated cells have specific functions and specialized structures, while undifferentiated cells are more general and have not yet developed specific functions. In short, highly differentiated is the opposite of most undifferentiated because it implies specificity, complexity, and distinctiveness.