I personally haven't flown from SFO/LAX to Asia before, but I have pretty much flown most airlines that do trans-Pacific flights from New York including Northwest, United, American, Cathay, KAL, Asiana, China Airlines, EVA, Singapore. Singapore is the best by far for economy, but CX is a close second. My only pet peeve with CX is that they don't have video on demand like Singapore in economy class, only continuous 45 channel video play (they have it in business class though). I just flown from NY -> HK -> Cebu 2 weeks ago on CX. NY <-> HK was pleasant, CX always use their newest planes on the longest flights so I am sure SFO/LAX -> HKG flights will use the new airbuses. But I agree with Pakman that on the short haul flights they use older planes (still newer than let's say, the AA flight from NY -> Tokyo I have been on). I was still impressed with the fact that CX served a real meal on that 1 1/2 hour flight - so used to US airlines only serving you one coke on a 6 hour flight between NY - SFO (United sucks!)
Don't bother with China Airlines, they get ex-Taiwanese military jet pilots to fly their planes. Safety record is worse than your other 2 choices.
My experience with KAL has not been so great in the past (older planes, bad in-flight food). But recently they are advertising on the Economist that they now have video-on-demand on their new flights, so someone else who has recently flown on KAL can shed some light?