Are you referring to hard product, soft product, or both? And business, economy, or both?

Delta is flying the A330-900neo on this route. I think in terms of hard product, some people would prefer the suites to the CI A350.
In coach, DL has an extra inch of pitch and a wider seat than CI or JX.

Not discussed thus far, though, is the difference in schedules. Delta is shifting its flight from leaving TPE early evening to one leaving at 9:30am come the end of this month-- making it hard to connect from other places in Asia. The other carriers all run late evening flights, which are better timed for connections from other cities in Asia, but worse for connections in the US --arriving in Seattle after 7pm.

Shifting the ex-SEA flight to later in the day will also likely be helpful for traffic from other parts of the US. The flight currently leaves before noon, but will be shifting to a late afternoon departure. But again, it will be poorly timed for connections in TPE - arriving at 9:15pm. The other carriers all leave SEA very late at night/early early morning and allow connections into the morning banks of flights at TPE