One thing I've learned about the Middle Ages is that it was not how I imagined it to be. That the geo-political structure was far more complicated than the simple pyramid taught in school. This makes me wonder if any medieval society actually met the stereotype.
France. England to the extent the French-Normann system was imported. But primarily France.
The best evidence I can bring for this is language. The language used to describe feudalism consists largely of French words. "Feodal" was first used in French legal texts and the word itself probably comes from Frankish. Terms like vassal, fief,feoffee, seigneurage, homage, liege all come from French.
It also seems relevant that the earliest and most famous historians of feudalism, such as Marc Bloch, Georges Duby, François-Louis Ganshof, are French or Belgian, focusing there. (There also includes Francophone Belgium, Valloonia, it was under the French crown back then.)
The best approximation of feudalism is a combination of Frankish and Roman laws regarding property, and it is debated but some medievalists argue Charles Martel created vassalage.
I should also add that many historians know reject the concept of feudalism as something not really useful. But as far as popular views of feudalism go, it is France, and Franco-Normann England.