The country with the largest percentage of European heritage in its population is difficult to determine, as it depends on how one defines "European" and how one measures ancestry. However, many countries in Europe have a very high proportion of people who identify as ethnically European.
Outside of Europe, countries with high percentages of people of European heritage include Argentina, Uruguay, and Canada. In the United States, about 60% of the population identifies as White, which is often associated with European ancestry. However, it's important to note that "White" is a broad racial category that includes people from many different ethnic backgrounds, and not all people who identify as White may identify as European specifically.