Best evidence connotes primary evidence; the best evidence available. Evidence short of this is "secondary.
A rule requiring that a party must call the best evidence that the nature of the case will allow. About 1700, the courts in England adopeted the rule that “the best evidence must be given of which the nature of the case permits.” ….. [Ford v. Hopkins, (1700)1 Salk 283]
This means that no evidence shall be brought in which presupposes still greater evidence behind in the parties’ own possession and power. And this is known as the ‘best evidence’ rule. Primary evidence is the best evidence affording the greatest certainty of facts. ….. [Abdul Mojid v. State (1987) 39 DLR 414]
Best gives three applications of the rule, namely: (a) Evidence to be receivable should come through proper instruments. (b)Evidence must be original and proximate. (c) There should be an open visible connection between the principal and evidentiary facts. And these are illustrated by the Evidence Act, 1872. The first rule may be found in s. 65; the second in ss. 50, 61 and 65; and the third in the theory of relevancy incorporated in Part I of the Act.