Most of the aforementioned Japanese places are what are recommended around Costa Mesa. The ramen and udon at Mitae Ramen are pretty good too.
Not as much good Chinese food recommended in the area, except maybe Golden Dragon near the southern end of Harbor Blvd. Pho 99 serves some pretty good pho, but that's about it.
Some of my favs are Santoka Ramen in the Mistuwa Market on Paularino.
Ango Tei is in the center right next door.
Oki Doki on Bristol is terrific
Anjin in the same center is also wonderful.
Mentasu Ramen is on Baker and Bristol has a good selection of Ramen, but I like Santoka better.
These are all lunch spots for me. For casual and fun dinner, it is hard to beat Honda-Ya in Tustin which is only 15 minutes away.