Started with my first job in 1981. Based in DTW so flew mostly NW. In 1984 took a job in ABE that required a lot of travel, continued to fly NW to Asia and Europe but US domestically. Eventually became CP on US but abandoned them for UA when their product went to hell. Was 1K on UA for many years and used them almost exclusively until I retired.
So for me,
NW->US->UA