Southwest Airlines, by far, carries the most passengers in and out of Las Vegas. In 2004, of the 41.4 million passengers who got off and on airplanes at McCarran, roughly 13 million of them were carried by Southwest (nearly a third). It doesn’t matter how you average them -- daily, weekly, monthly, annually -- SWA wins the contest, hands down.
Its nearest competitor, America West, transported approximately half as many passengers -- 6,828,903 -- in 2004. The other three "top-fivers" were United (3,186,979), Delta (2,780,046), and American (2,594,439).