Best Meal Replacement Shakes

What we eat, when working out to either gain or lose weight, is pretty important. In fact, half the battle is won or lost, based on what you eat. Say you are looking to gain muscle, what should you eat? Should it be fat or protein based? And how about weight loss? Ok, no fat there. But you need some source of energy, so how to maintain a balance between the carbs and the proteins? Confused