Which US Airline Has The Highest Passenger Load Factor?

For decades, the Boeing 737 has been one of the defining aircraft in short and even medium-haul markets. However, its operational importance is best understood by the airlines that have built entire network strategies around the type. From Southwest Airlines’ to Ryanair and its ultra-low-cost expansion across Europe, the aircraft has proven itself as a key workhorse for low-cost operators. So which airlines are the largest operators of this jet?