TG direct nonstop is the best choice if you have no interest in stopping over anywhere on the way. You're guaranteed the A345, and the economy product (esp seat pitch) is good. Premium Economy is excellent product if you can find a deal (tough this year, it seems). Asiana via ICN is longer but also good if you get the 747--the older 777 product that is also used on this route is not as good in Y class--so check what's flying on your dates. ICN is a nice transit point. I don't like how NRT/HND routings still leave me with a 6+ hour flight NRT-BKK, so I'd rank OZ as a preferred choice, though sometimes connections there can be overly long.

Some people rave about SQ but when you look carefully, it's the first and biz class experience. There isn't that much to distinguish it in Y class except for IFE system, and the seat pitch is meaningfully smaller on SQ. On a TPAC flight, seat pitch is probably the #1 determinant of sanity, outweighing food quality, IFE, etc. I wouldn't add extra time to my journey by going to south to SIN then back north to BKK. Nonstarter for me unless flying in a premium cabin. I find SQ fares to be relatively expensive for econ class and poor value for money.

I also haven't heard great recent reports on Cathay in econ TPAC, so your instincts to deprioritize them are probably correct. You might also look into EVA via Taipei connection, though it will be of limited use if you are collecting miles. EVA has a prem econ product also. China Southern has a premium economy often coupled with very competitive fares, transit in CAN (6 am arrival in CAN, 8:50 dep to BKK which is doable transfer). No Chinese visa needed for this intl-to-intl transit. This would be the only mainland connection I'd consider from LAX-BKK if you have no interest in stopping over. CZ is Skyteam. If you have an interest in stopping over in Shanghai (US passport holders get 48 hours to transit with no visa needed and yes, you can leave the airport and go to city/stay in city), then American, United, China Eastern can get you there. China Eastern flies on to BKK on their own metal, and also codeshares with TG on TG metal on PVG-BKK.