United lands at the bottom of the list. The only surprise here is with myself; I've somehow resisted the urge to call Delta to ask if I can transfer my United miles/status to Delta's frequent flier program. But trying to switch your frequent flier status and miles from one domestic airline to another is like trading a French franc for an Italian lira or a German mark — they're all worthless.
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Delta > American Airlines > United.
I finally weeded United out of my international choices. I have lots of miles on both Delta and American. Delta was also nicest about gender change. United was meh. American was horrific. I wrote about that on GT.