The latest to arrive mostly.
In a way America is the melting pot we claim it to be, but we have a habit to turning the fire up when we add new ingredients. America puts its newest through hell, but in a generation or two, they are generally excepted. The Irish and Chinese are great examples of the way we have mistreated new immigrants and then accepted them. When a group comes here illegally, such as many from the Central and South American areas in recent years, its different. They have more to overcome than just newness.
But another answer to your question would be that reservation resident Native Americans and long term generational Black citizens are the most misunderstood and misjudged.
The first group still suffers some people's hatreds and bigotry, possibly because they may still be identified with their ancestors as a conquered people. The fact that reservation living keeps them separated, not part of the melting pot, with different rules and different services (mostly substandard to what other Americans have and therefore in what can be considered a perpetual state of underachievement and poverty) keeps a sense of other going long past any need. I have only seen this attitude toward natives in non-native people living around large reservations. A person that meets a Native American elsewhere in the country doesn't seem to have the same biases.
The second group has suffered a bigotry that has no explanation. It has outlived all reason for existence. The sense of other involved here doesn't extend to say, Caribbean blacks or recently emigrated Africans. Their accents seem to exclude them from same treatment. But long generational Black Americans are judged more harshly and treated to more suspicion. Some of it is a kind of judging all collectively for the sins of a few, but we don't judge other cultural groups with such a harsh yard stick.
I don't understand your saying this excludes racial and ethnic groups. We are all members of racial and ethnic groups. Race and ethnicity are a large part of what creates cultural divides.