At this website (document `Federal debt', table FD-5) I see that more than 16 trillion dollars of US debt is owned by private investors. Is there a list available of the private investors who own the most US debt?
No there isn't any list of private US debt holders. When someone purchases bond that is a private information. Some people might publicly disclose it but there is no list.
Private companies that are not public corporations also do not need to disclose their financials so there is no way of knowing in most cases.