Depends on the era.
In the modern era, the answer must be Death Certificates, which are usually loaded with dates and places related to the deceased, his/her parents, spouses, and burial locations. Even identifying the informant can provide clues about the family. A few give sparse or misleading info, but most are chocked full of goodies.
But, death certificates are a recent invention. Prior to that the best general source is probably Census Reports. Wills and Estate Suits give much more specific information, but not everyone left a Will or an estate. Many people disposed of their property within their lifetime... the only way to insure one's wishes are implemented. Census records are found for all families and provide critical information for families who lived in remote areas with few legal documents. I love Wills, but we'd be mostly in the dark without Census Reports.
Prior to Census Data, it must be Estate Records. After that, anything goes... Court Records, Tax Records, Deeds, Church Records, Bibles... and lots of Luck!