According to their website, Phoenix is their largest operation... here's the full list with the number of daily departures:
Las Vegas, 154
Los Angeles-LAX, 122
St. Louis, 85
The city with the highest number of nonstop markets served is Las Vegas, with 37. Also, none of these cities are "new": the last one to make the list, Baltimore, was established in 1993. Most are from the early-to-mid 1980s.