Either collectively, or on an individual basis?
I don't think there can be any serious discussion that LA does Chinese better than any city in the U.S. (and maybe North America with the exception of Vancouver and *maybe* Toronto). Flushing and SF are pretty good, and I'm told that Houston is a serious up-and-comer, but all of those places bring up a distant rear.
And with Japanese, I know that NYC has many many great sushi places -- esp. the high-end variety -- but does NYC Japanese span the spectrum that LA (and Orange County) can offer in terms of things like monjayaki (or okonomiyaki), kaiseki, ramen and soba, izakaya, kare raisu, etc.? I don't think so, but my Japanese is limited and I'd be curious to hear from those who have a deeper and more intimate appreciation of all things Japanese.
I think the same could be said for Korean. I think if you included all the Boroughs and NYC, the Korean might be comparable with what you can find in LA, but it still lags a bit. And, as with Japanese, NYC has better high-end Korean (e.g. Jungsik, Bann, Danji) but for overall depth and diversity I think NYC (w/o counting the Outer Boroughs) is not comparable to LA.
And I think collectively, there is probably no city in the United States that's in the same league.
But I'd like to hear people's thoughts. And I don't want, nor mean, this to be an avenue to denigrate any one city, or region. Just because we can applaud one city, does not mean we have to be heckling another one simultaneously.