An Airbus A330-323 aircraft, operated by Delta Air Lines.Benoit Tessier | ReutersDelta Air Lines is the country's most profitable airline. CEO Ed Bastian's challenge is to make sure his carrier stays on top.The airline's unit revenue, the amount it brings in for every seat it flies one mile, outpaced its competitors' last year. Delta's share price has soared almost 23% in 2024, more than any rival in the rocky airline sector, in a rally that's outdone the S&P 500's. It expects free cash flow to rise as much ...