Second Joyful House, but there are many good places in the area. I like Harbor Palace, 4275 Spring Mountain in Chinatown itself; Sun Chinese Gourmet for cheaper fare at 4711 Spring Mountain Road; and K J Kitchen, 5960 Spring Mountain Road.
I went to a newly opened restaurant in the plaza where 99 Ranch is located. It is upstairs and to the right of the staircase. It is some Shanghai style place, but it turned out to be really, really bad. So, don't try it, and service is awful too!