Who are the big six USLC clubs

The Premier League having a big six is quite unique actually, most leagues internationally have a big two (e.g. Scotland, Turkey, and Spain) or a big three (e.g. Portugal, Italy, Netherlands, and also Spain). The rest tend to be dominated by a single club (e.g. France, Austria, and Germany)

*Spain is mostly dominated by Barcelona and Real Madrid, but Atletico Madrid pops off like once every ten years and wins