IndiGo, India's biggest airline, has become the world's worst-performing airline stock just two months after a blockbuster initial public offering. The carrier's operator InterGlobe Aviation fell 25 percent in the four trading sessions through Jan. 25 -- the worst compared with major airlines as well as the 48-member Bloomberg Intelligence Global Airlines Competitive Peers index. A potential delay in receiving new fuel-efficient jets is said to have hurt the stock.