What are the safest comforters babies can use? Thank you

Red Nose does not recommend the use of soft toys or comforters, especially for babies less than 7 months of age.

Keep soft toys out of the sleeping environment for babies under seven months of age because they may cover the nose and mouth and interfere with breathing.
The risk posed by suffocation by the presence of soft objects in the baby’s sleeping environment outweighs any benefit to the baby from a soft toy. It is therefore advised not to place soft toys and other soft objects in the cot for babies under seven months of age.

Red Nose does have the following information about soft toys/comforters for older babies.

Seven-month-old babies are more likely to explore objects in their sleeping environments than younger babies​Some babies over seven months of age may appreciate a small object such as a soft toy to provide comfort and connection (transitional object) during times of separation from their parent.
https://rednose.org.au/article/soft-toys-in-the-cot