Word or expression for a constituent that perfectly exemplifies the whole

Solution 1:

I think a great word for you is Epitome

You would rewrite your sentence something like:

The Godfather 3 is hopelessly bad for many reasons; this scene is the epitome of everything wrong with the movie.

epitome at Oxford Living Dictionaries.

1. a person or thing that is a perfect example of a particular quality or type.

"she looked the epitome of elegance and good taste"

synonyms: personification, embodiment, incarnation, paragon; More

2. a summary of a written work; an abstract.

Note.. the most common uses of this word is to call out positive traits but I've certainly heard it used for negative summations.

Solution 2:

Quintessential

representing the most perfect or typical example of a quality or class.

Embodiment

a tangible or visible form of an idea, quality, or feeling.

Usage:

The Godfather 3 is hopelessly bad for many reasons; this scene is the embodiment of everything wrong with the movie.

or

The Godfather 3 is hopelessly bad for many reasons; this scene is quintessentially everything wrong with the movie.