SQ has the best service, clean restrooms and is the most comfortable due to the 3-3-3 in Y, but you will have to take the 2 hour hop down to SIN. Worth it IMHO.
BR (EVA) used to be my “go to” choice until they went to the 3-4-3 set up in Y, with only a 17” seat width. If you are a thin framed individual, EVA is a great choice with great service, very clean restrooms and comfortable new seats (just narrow). If you can find a good price for their “Elite Class” premium economy it is well worth it.
Another option is NH (ANA). Again narrow, but comfortable seats in Y, great service and clean restrooms. ANA also has the option of flying into SJC (San Jose) if your final destination is in the South Bay, Silicon Valley or Monterey.
All 3 of the above are Star Alliance and can credit to your UA account.
KE (Korean Air) is a comfortable, wide seat in Y, but they run a very warm cabin temperature that has made them a no go for me.
Happy travels!