Personally, I'd pick Cardassia. Since the Dominion War ended and they joined the Federation, there's going to be a lot of business and diplomatic traffic. But they won't pay high fares since the Cardassian economy will take time to recover. Also, Cardassia might generate good tourist traffic for outdoorsy types. The weather's nice, and Risa is getting overpriced.
Finally, the success of Southwest's Kronos station indicates that they do well on planets with warrior cultures. WN
's employees are famously good at getting Klingons to hand over their bat'leths as checked baggage.
Barring Cardassia, I'd look for COS
as good possibilities. MKE
goes high on the list, though, if anything happens to Midwest. TTN
's wealthy NIMBYs and short runway will probably keep WN