What is the best example of a primary source document useful in the study of history?
There are a number of possible answers.
If you mean what is the best example of a primary source document in the study of history, then you are looking for a source which is impartial and objective. This allows the researcher to use it as a valid and reliable source of information from which to draw conclusions.
This could be observations and diaries from a given time in history e.g. Pepys's diaries. They give a personal and detailed insight from someone who lived at that time. However they could be riddled with bias reflected in the observations of the individual thus undermining their validity and reliability.
A second primary source could be a general history from the period, e.g. Tacitus' account of his father-in-law Agricola's campaigns in Britain. Again this gives us an insight from that time but, also again, the account may be highly biased. In this case someone writing about his father-in-law at that time is unlikely to be critical.
A third source is imagery. Early examples would be photographic records of the Crimean War and the American Civil War. In the former such imagery along with the reports by Russell reporting for The Times were instrumental in changing public opinion about the nature of warfare.
A more recent example would be television coverage of the Vietnam War, the first and last extensive TV coverage of conflict. In both instances what is being shown in terms of imagery is metaphorically and literally a snapshot. As with all sources both primary and secondary they have to be treated with caution.