If the criterion for flat is less than 10 feet of elevaton change, Riviera definetly doesn't qualify; there is considerable slope from the 17th tee to the 18th green.
I'll nominate Kittansett Club; the only elevation change there is in the mounds, which are rock piles covered with earth. The course is suprisingly strategic despite the dead flat site. The mounds in concert with sahara bunkering on the par 5's provide all the interest one needs when the wind blows. Even the majority of the pushup greens, especially the ones most exposed to the wind, have little height. The greens on the interior of the course are the most interesting. Although the course is surounded by trees, the scrub oak and pines are no larger than 20 feet tall, increasing the effect of the sea breeze.