Binary Representation of Everything Else

For every type of information stored in a computer, we need a code mapping (or encoding) which defines how items of that type will be represented in binary.
Representing characters: by far the most common scheme is the American Standard Code for Information Interchange (ASCII).
What else? We need good encoding for real numbers , machine instructions, pictures, sound, movies, etc., etc.
Question: What makes one encoding scheme better that another?