Cotton is one of the best all-around fill options for a duvet insert. It's lightweight, breathable, and easy to clean since the duvet cover filling can be washed in a washing machine and dryer. This is a great choice for those who need more durable bedding for kids and pets.
If you're looking for something more luxurious, down and feather-fill options are perfect for their warmth and fluffiness. These are made from duck or goose feathers and are a great choice for people in colder climates. They also tend to have higher fill power, which means they are lighter, more voluminous, and offer more warmth overall.