Technically, colonialists are not “immigrants”. Regardless, if we go by first settlers then it’s the English since they settled first in 1587. First German settlers didn’t arrive till 1628, and when the U.S. was formed these disparate groups combined together to form a better front for fighting. Germans fought on both sides. If these different settler groups didn’t have a common enemy (the Natives or English), they would have started fighting each other. In fact, that’s why we still see remnants of the US civil war to this day.