My vote would go to Spirit, which is still seeing the biggest loss margins in the industry. Although their CEO claims new fuel efficiency will save the day.

""Can Spirit survive in a world of JetBlues and Southwests? I would say, for at least the short term, yes," said Alan Bender, a professor who specializes in the aviation business at Embry-Riddle Aeronautical University. "But it appears the market is nearing the saturation point. There will be consolidation at some point."

The strains are already evident for some low-cost carriers. Independence Air folded last week after parent company FLYi Inc. filed for bankruptcy. Delta Air Lines Inc. shut down its low-cost arm Song.

Ray Neidl, an analyst with Calyon Securities, said that Spirit is well positioned by replacing its jets with a new, more fuel-efficient fleet, but passengers aren't necessarily loyal to discount carriers because "they just want to get the cheapest ticket possible."

The airline is switching its aging fleet of gas-guzzling MD-80s for 31 Airbus jets sporting leather seats and movable headrests in all classes. It expects to complete the $2 billion refurbishment program by September.

Spirit posted $33.7 million in operating losses for the first nine months of 2005, larger than the $16.4 million in the period in 2004, according to the U.S. Bureau of Transportation Statistics. Baldanza also expects a loss in last year's fourth quarter.

"Most of the losses of the year are tied to the fact that we're still flying old airplanes and we're spending a lot of money to buy the new airplanes," said Baldanza, who joined Spirit from US Airways in January 2005. Spirit also flew less last year because of scheduling conflicts caused by the transition to new jets.

Baldanza said that Spirit would have about broken even if it had an all-Airbus fleet in 2005. The Airbus jets burn about 20 percent to 30 percent less fuel than MD-80s on a typical flight, he said. Spirit has also hedged its fuel costs through this summer, although to a lesser extent than Southwest.

"2006, we expect, is going to be a much better year for the airline," he said. "I think we should make money in 2007 and be a very exciting profitable growth story as we go into '08 and '09.""