What is the best treatment for a scratch on the cornea?

See ophthalmologist. Corneal scratches are quite painful. They can rarely get infected if the agent causing the scratch was contaminated. However tears are very anti-bacterial which helps. Treatment includes topical moisturizers, a therapeutic contact lens in some cases and anti-pain medication if needed. Uncomplicated scratches develop a new skin in 24 hours typically. If longer, see an ophthalmologist.