Blacks by a long shot. Mexicans have issues too but at least they have a home country that supports them, even if it's a relatively poor one.
And the hierarchy of racism is White > Yellow > Brown > Black, which is reflected in how much money each group controls in this country. This is intentional because of the fact that historically blacks have always shown that on an equal playing field, they will almost always shine the brightest.
There's also the fact that blacks hold the highest genetic potential to pass on their traits because they have the most melanin, but that's probably for another thread.