1. It emits the maximum light we can see in the solar system.
2. It is the source for heat. By that source only most of the planets could be equilibrium in their temperature.
3. It supplies the required energy to the plants so that they can produce the food.
4. The Sun heat is required for raining. It evaporates the water on the surface of the Earth and make them in to the Clouds.
5. It helps us to get the required amount of Vitamin-D.So as we are not getting the diseases.
So I think these are the reasons why Sun is most important in Solar System.