You've changed from “perfect” to “representative"; while quintessence can mean either, the latter makes little sense with “the” [0] only “a”, the former makes perfect sense with “the” to refer to the concrete example best approximating an abstract ideal, and this is exactly how the construction “the quintessential X” is generally used.

[0] except in the context of some feature where the example perfectly represents the aggregate in a way no other single example does.