The best diet for a ferret consists of high-quality, commercial ferret food that is specifically formulated for their nutritional needs. It is important to look for a ferret food that contains at least 30-40% high-quality protein, around 15-20% fat, and is low in carbohydrates. Additionally, you can supplement their diet with fresh meat, such as cooked chicken or raw bones, and occasional treats like eggs or fruits. It is important to avoid feeding them fruits high in sugar or dairy products, as these can upset their digestive system. Always provide fresh water for your ferret to stay hydrated.