incline works the top of your pecs, which is one of the hardest muscles to develop, and decline works the bottom part of your pecs. you can get good looking pecs by just doing flat, but they will look better overall if you do all three
The outer chest will develop the fatest. This comes from flat bench. Start chest day with inclines. Everyone likes to throw up alot of weight on flat bench, but my chest has shown most improvement by focusing on incline. Also focus on the upper squeeze on these muscles and make sure the angle is not too high or you'll work shoulders. Move your grip closer to work inner but not too close or you work triceps. cable crossovers will give a fuller look over pecs and can increase the definition of the center cut once there is enough mass.