The correct answer is - Hawaii region.
The Hawaii islands, have the rainiest place in the United States, it is the Mount Waialeale on Kauai, and it averages around 11,684 mm of precipitation. This actually makes this place, not just the rainiest in the United States, but it is up there in global terms as one of the rainiest places in the whole world.
Most commonly, the northwest or the central south coastline are seen as the rainiest ones in the country, but people very often forget that the Hawaii are actually part of the United States as well, so make an honest mistake.