Since I am a diabetic, I don't drink fruit juice. It is more important for me to eat the whole fruit. But if diabetes is not a concern for you, it is suggested you drink juices that are high in vitamin C during cold and flu season. Vitamin C is important in helping the immune system function well. Studies show that people who regularly get vitamin C in their diets have shorter colds with milder symptoms*. Juices suggested by doctors include orange, grapefruit, and papaya.
*Note this says, "Who regularly get vitamin C in their diets". This is not a recommendation for taking vitamin C tablets, etc.