Simple question. What is the cheapest airline?
As much as I'd like to help, that is a "simple question" that can't be answered.
"Cheapest" depends on a lot of factors.... departing airport, arrival airport, how far in advance you book your travel, what kind of a plane would like to fly on, do you want to fly non-stop, or are you willing to make one (or more) stops or plane changes. I'm sure there are other factors that I'm not listing.
For example, if I want to fly from my home in Little Rock to Dallas, I can choose either American or Southwest to fly non-stop (also Delta but I usually don't fly them). All of the airlines tend to match each others' fares and sales on this route, so price is the same. On Southwest I get a larger plane (737), but I don't have an assigned seat. On American I get a smaller plane (50 or 70 seat regional jet) and an assigned seat. Service is the same on both, a beverage and a bag of snacks.
If I want to fly from Little Rock to Baltimore, I can take Southwest non-stop (737), or I can take American, Northwest, Delta, USAir and change planes in either Chicago, Memphis, Cincinnati or Charlotte... and at least one of the flights on any of the four airlines listed above will be on a regional jet. Again, in flight service is basically the same.... a beverage and a snack.
If you live where one of the "Low Cost Carriers" (Frontier, jetBlue, AirTran) has service, you may get in-flight entertainment, such as DirectTV or Satellite Radio.
I wish there was a simple answer to your simple question, unfortunately, there isn't one.