Southwest- Cheap and cheerful, great service for a budget airline, nice selection of routes & destinations and great value with little difference on 1 or 2 flights.
ATL/HOU- Great airports. ATL was easy to navigate despite its size & reputation, HOU was a nice easy means to make a connection. Neither too busy given the times I flew.
DAL- Definitely a hidden gem for Dallas! Small but busy for traffic, great museum just off the airport, A lot quieter and better to use than DFW, plus great transport connection into the city!