Why does the CG look so good on the JPN and asian trailers but so average on the US trailers?

I haven't noticed much of a difference to be honest

Nor have I.

There definitely is a difference. If you do shot by shot comparisons of several scenes you can see it, although slow motion may be required. 

One of the most notable examples is the shot of Godzilla rising out of the water near the island during Serizawa's "In 1954, we awakened something..." bit. In the Japanese trailer, Godzilla's color is different, a much darker grey, the spikes seem to be at a higher resolution/detail, and the water spray around him is thinner, and more of a mist, rather than a thick white splash. 

And that's just one of the differences. People already pointed out the Muto that was added into the Vegas destruction shot. There are some others as well.

I don't think it's Asian favoritism or anything like that. I just think the Asian trailers have come out later in the production cycle, and therefore have had greater benefits of post production editing. If we wanted the U.S. trailers to look as good, we probably would have had to wait longer for them. 

Just my 2 cents anyway.

I thought it was imagination, but I also recognized that.

I think it's the image format, I also think the Japanese Versions have a higher resolution, a true Full HD, the US trailers look more like 720p HD ready.

I have no doubt there are probably differences, but nothing that sticks out too much. It all looks like standard, western effects work of post-LotR era Hollywood to me. Nothing exactly genre defying or visually different from anything else. Same 'ole, same 'ole out of western visuals.

Which is fine. That's kind of the point here I suppose.

Yeah. Asia got the "finished product" trailer. Theres a fine difference indeed.

I think the Japanese trailer looks better than the others. But I don't really see the difference between the US ones and the latest asia one.

There's always that line that fimmakers/studios have to straddle - Fans want to see something ASAP, but the problem is the effects work is nowhere near completed. They release teasers/trailers and fans bitch about the less than stellar effects! Yes it's very possible the CGI is slightly better, because they're still working on them, and with a little over two weeks to go probably still are. They'll tweak what they can with whatever time they have left. I can't wait to see this!

No MUTO in the Las Vegas shot, though in the Asian trailer... Interesting.

I think there was only 1 trailer/spot so far where you see the walking MUTO in the background of destroyed Las Vegas, all others (the new Asian trailer included) have the old scene without MUTO walking...