What is the most important function of chloroplast?

The most important function of chloroplast is converting light energy into chemical energy by the process of photosynthesis. 

Chloroplasts are specialized organelles found in plant cells. They are characterized by their high concentration of chlorophyll, a green pigment that is capable of capturing solar energy. Chloroplasts use the captured light energy along with carbon dioxide and water to produce adenosine triphosphate (ATP) and Nicotinamide adenine dinucleotide phosphate (NADPH). ATP and NADPH molecules are used to synthesize sugar and oxygen.