What is the best example of a primary source document useful in the study of history?

It does depend on the type of work you are doing and the sources available. For example, in a text book, if there is a primary source within a textbook/book with other information, then you could count that as a primary source, but how you reference in footnotes would be different to other types of primary source. It also depends on the time period in which you're writing about as to what primary sources are useful. For example, if you're writing about the portrayal of a certain element of British history in the 1980s within TV, for example, one of your best primary sources would be to use clips from popular television shows during that time. How you source them is another matter. Please contact me if you'd like more help on this!

Different fields and scopes of historical study use different primary sources. Sometimes it helps to use new sources to expand the field of study. And sometimes certain sources aren’t available (were either destroyed or just not kept). So there is no one single ‘best’ source that fits every study. However, traditionally written sources you can find in archives are considered the best to use.