I think shampoo is more important. It cleanses the hair and removes any build up of dirt and residue that reside on the scalp and hair through mild detergents and by opening the cuticles along the hair shaft. Throughout this process, some moisture and natural oils that are essential to healthy hair are lost and that is where conditioner plays an important role.
I believe they are equally important. Shampoo cleanses but it also strips all the natural oils that your hair needs. (without these oils your hair would be brittle & would break easily) Conditioner replenishes these oils.