Argentina and uruguay are clearly the most European countries in Spanish speaking America, but the Spanish are perhaps eclipsed by the Italians in terms of their contribution to the ancestry of the people in the country.
Also, while I’ll agree that Buenos Aires is essentially European, some of the back country of Argentina (around the Andes) is pretty heavily mestizo, though Bolivian and Chilean immigration is part of this.
I would make the case that if you are talking about actual Spanish (as opposed to European in general) ancestry, (pre-Castro) Cuba would have the highest percentage. Some figures I have seen claimed Cuba was 75% Spanish in the 1950s, though more accurate figures might be closer to 50-60% then, and probably 30-40% today. One factor is Cuba and Puerto Rico remained Spanish colonies up to 1898, and retain a Spanish element. Chile and Costa Rica are mostly mestizo, but there is a stronger Spanish element in mestizos from those countries than in many others, mainly because the indigenous people put up more resistance and were driven away from much of their land. So the mestizos there are often more European looking. By contrast, many mestizos are actually the descendants of indigenous people who adopted Spanish ways and ceased to be “Indians”.
Different Latin American countries had different patterns of settlement and intermarriage. Peru and Mexico have a similar history in many ways, but they are still quite different because in Peru, the Spanish mostly settled (along with their African slaves) on the coast. The Native Quechua and Aymara stayed largely intact in the highlands. So currently Peru is about 40-50% indigenous, 40% mestizo (including many Hispanicized Indians), and there is a white population concentrated around Lima, with Asians and Blacks. In Peru being “white Peruvian” is almost synonymous with being from metropolitan Lima
By contrast “provincial” is often a euphemism for mainly Indian blood. Of course Lima today has many people from the interior of Indian ancestry.
In central Mexico, the Spanish didn’t start a colony alongside a conquered Indian empire, they built one on top instead. So the Mexican capital probably always had a predominantly Indian or part Indian majority, and the “white Mexican” population is a very small elite mostly in the cities, but not as separate from the majority as in Peru. Mexico now is more like 10-20% indigenous, 70-80% mestizo, and probably 5% more or less European.
In southern Mexico, around Chiapas and Oaxaca, a “Peruvian model” of settlement took place instead, and as a results those states have large Indian majorities. And in Northern Mexico, the Native population was more sparse (Apaches and Yaqui as in Arizona and New Mexico), so little mixing between Spaniards and Indians took place there. Settlers from further south brought most of the Native ancestry. Some of the people from northern New Mexico, who are not Mexican immigrants but the descendants of Spanish settlers, are often very European looking.