The cheapest one.
But being real, if I had to go back again (her grandma has since moved to Las Vegas to be in assisted living near my sister-inlaw) I'd go first class. I could tolerate the long flight if I had some room to lay back and stretch out. Both my in-laws are pilots for Southwest based in Vegas and they obviously swear by them. If you want to get drunk on a plane, do Southwest. Personally, I like me some Delta.