I'd agree with Koloa Fish Market & Fish Express. Also, though not plate lunch, Hamura Saimin in Lihue Town for saimin and excellent manapua (chicken & beef sticks are ono too).

Da Imu Hut in Hanapepe is another local's favorite & Ishihara Market in Waimea town (on the way to Koke'e) makes a variety of dishes that are very good as well.