What airplane would you take to Las Vegas? Frontier, Southwest or U.S Airway?

I would take whichever is most convenient and/or gives the best price. While Frontier is a discount carrier, the extra fees can add up quickly. I like Southwest a lot and fly with them when I can, but they don't fly in and out of my local airport.

None of them as those Airlines do not serve an Airport near me to Las Vegas.
I would take a direct flight from one of two others.

Well, not USAir, unless you’re time traveling.

I would take Southwest anywhere over those other two. Their fares are lower, they have the best reviews, and they have the best on-time records.

One thing though...you do not get reserved seating. So you need to know how to play that little game to get good seating aboard their airplanes. It can be done, getting good seats, but it's tricky. If you know someone who flies Southwest regularly have them show you how its done.

It depends on where you are flying from. Also US Airways is now part of American Airlines. I like that Southwest allows 2 checked bags to fly for free but not a fan of unassigned seating. Southwest would have the highest customer satisfaction rate among the 3 airlines mentioned.

Southwest, since they have a hub nearest to me that I like to fly out of.

If all offer an option from where you are flying from, it would be down to price. US Airways (essentially American Airlines) might offer a snack and a drink complimentary, the other you may have to pay. If it's a couple of hours flight I doubt there is much in it.