The investment case for buying stock in the airline just got a lot stronger.Delta Air Lines (DAL -0.45%) recently released earnings and the results were excellent, checking off nearly all the items many investors look for from a company in the transportation sector. As such, the stock remains a great investment option for those seeking a long-term growth stock.That's the overarching take on Delta. Now let's take a closer look at why this is the best airline stock to buy at the moment.Delta ticks all the box ...