Answer: The cell type in a leaf that performs the most photosynthesis is the palisade mesophyll cell.
Explanation: These cells are located in the upper part of the mesophyll layer of a leaf and are packed tightly together. They contain many chloroplasts, which are the organelles responsible for photosynthesis, and have a large surface area to maximize the absorption of sunlight. The palisade mesophyll cells are also located close to the upper epidermis of the leaf, which allows them to receive the most direct sunlight. Therefore, they are the primary site of photosynthesis in most plants.