Caveat: This is just from personal experience flying 2-3x/year on the routes you mentioned (E/W Coast N.Am - S.E.Asia) in the 90s-2010s in Economy/PE.

The segment between E.Asia and S.E.Asia is totally fine in Economy with any of these airlines.
For the N.Am-E.Asia segment, I’d rank my preferences and reasons as follows:

JAL, ANA: mainly for the level of hospitality and cleanliness as in Japan, slippers, and bidets in the bathrooms. They often offer upgrades from Y to PE at a reasonable price at the airport. In case of delays/cancellations, they’ve always accommodated me well in Tokyo.

SQ, EVA, China Airlines: For China Airlines, YVR-TPE route especially when they operate a 747 and you could book Economy and select the regional business class seats upstairs — this may no longer be the case though. EVA’s PE may not be that much more expensive than Economy, depending on time of booking.

KE, Asiana, Cathay: This order of preference is food-based 😂 Otherwise: Cathay, Asiana, KE.

In general though, I do find the newer seat cushions across all cabins to be less comfortable (harder and thinner) than the older softer ones back in those days. So find routes with older cabins if that’s important to you. And it’s worth it to select extra legroom seats. And yes, do try to find deals or upgrade to PE as with some of these airlines, the PE products and services are superior to even some US domestic first class or Iceland Air Saga class experience.

FF points: JL/CX/SQ/KE can all be credited to Alaska. ANA, SQ, EVA, Asiana to any *A program. (Asiana depending on when they stoop crediting to *A) KE and China Airlines to FlyingBlue or Delta.