Bear in mind the wheel also absorbs crash impacts, especially in off road, so a softer wheel is better in regard. Too stiff a wheel would simply crack/shatter or start breaking other parts.

I've had good experiences with Xray rims. ^Nylon is good as mentioned, but carbon filled can be brittle. Much better to do what AKA did and have a secondary stiffener plate. AKA PRODUCTS, INC. Wheel Stiffener: Evo Wheel, Yellow (4) : Truggy | Horizon Hobby