In most plants , photosynthesis occurs in the leaf which is the common site.
It is the best site because it has the highest amount of chlorophyll in most of the plants.
leaf contain stomata so they can easily exchange the gases.
in a plant the leaves have the maximum surface area and it can perform photosynthesis in a more efficient way as the rate of photosynthesis increases with the increase in the surface area available for photosynthesis.