What is a good word for “best example”? I would like to describe a film as the greatest example of its features.
I would suggest epitome.
(the epitome of) a person or thing that is a perfect example of a particular quality or type: she looked the epitome of elegance and good taste
Oxford Dictionaries Online
Dictionary.com defines paragon as
a model or pattern of excellence or of a particular excellence: a paragon of virtue.
EDIT: Current usage of paragon in the context of films:
...leading as it does to the most famous movie scene of all time, is held up as a paragon for subsequent generations of aspiring screenwriters...
Black Swan: (blog title) "Dramatic Film Paragon."
Here are 5880 references to the archetypal zombie movie, for example.
archetype: the original pattern or model of which all things of the same type are representations or copies : prototype; also : a perfect example.
Alternatives such pre-eminent, paragon, epitome, and even exemplar almost always have strong positive connotations. They don't work for a good example of something bad, such as an archetypal serial killer.
"A is the quintessential example of B."
quintessential - adjective
representing the most perfect or typical example of a quality or class: he was the quintessential tough guy—strong, silent, and self-contained
"Par excellence". Usually placed after, rather than before the category you are claiming it is the best of, as per its original French use:
Pretty Village, Pretty Flame is the gritty war-movie par excellence.
It's a "still foreign-enough" loan-word that many people would italicise it in use.
Pre-eminent might also serve, though it more reflects how a film is seen, and its standing, so if you disagree with the general public, or the film you are going to describe is under-rated in your view, then it won't work:
Cross of Iron is the pre-eminent gritty war-movie.
You can use the phrase "Exhibit A". Some other examples:
Try "paradigmatic example". E.g.:
§ Hot dogs are the paradigmatic [example of] American food.
§ It was a paradigmatic example of what happens when collective memory born of trauma finds political and, above all, military expression.
Try "textbook example" [cf.]. E.g.:
§ At first glance, Marquez’s actions were a textbook “straw purchase,” in which a person buys a gun for someone else ...
§ Pressed by a reporter on how he can support Trump in spite of his “textbook” racist comments, ...
He's pretty much a textbook case (example) of ACOA. [(in reference to the 13 characteristics)]
Try "exemplifying example" [cf.]. E.g.:
§ The plays of Wilde exemplify the comedy of manners.
§ This painting perfectly exemplifies the naturalistic style which was so popular at the time.
§ a dish that exemplifies French cuisine
His works exemplify the taste of the period.
§ the best dry sherry is exemplified by the fino of Jerez
§ A boiler room operation is well exemplified by the case of Fort Lauderdale-based company First Resource Group LLC charged by the U.S. Securities and Exchange Commission with conducting a fraudulent boiler room scheme in 2012.
Try "customary example".
Try "symbolic example". E.g.:
§ Jane Doe first moved 10 months ago to avoid deportation. Now she's become a symbol of paralysis in immigration reform.
Try "paragon". E.g.:
§ The Southern accents were molasses-thick, and the diner setting made the sitcom "Alice" seem like a paragon of comic subtlety.