Despite mixed performance relative to the market over the last few years, the airline stock is set to fly higher.Delta Air Lines (DAL 0.27%) stock has underperformed the S&P 500 (^GSPC +0.54%) over the last five years and the last year, but has beaten it in the previous three years. Still, the key question isn't so much where a stock has come from -- it's where it's going.On that basis, Delta Air Lines is one of the best stocks in the transportation sector, not least because it's likely to climb a wall of w ...