The Rockbros knockoff of Oneup composites are a great first (or only) flat pedal.
I have a pair of real OneUps and personally I can't stand how convex they are.
I would rather have something flat or concave like MZYHR 327 or some Shanmashi alloy pedals.
Also Marin composite pedals are $20 at Bikesonline right now, I like those better too.