The phrase ‘best evidence’ is used in legal proceedings and refers to the premise “that no evidence was admissible unless it was “the best that the nature of the case will allow“”. This means putting one’s best foot forward in presenting documents to the immigration department is mandatory whenever possible.
Common scenarios where this is extremely important are when trying to prove you perform a certain job, possess a certain skill, have acquired a certain amount of savings, have been victimized by a government official, have met the requirement to reside in Canada a certain amount of time, have been in contact with your spouse, and so on.
The often cited case that tries to moderate the above requirement to present the best evidence in a legal matter is the case of Maldonado v Canada (Minister of Employment and Immigration),  2FC 302, 31 NR 34 (FCA), which asserts that there is a presumption of truth in sworn testimony or an affidavit However, a recent case basically reiterates that a sworn statement from a client doesn’t establish the truth of what is being sworn to. In the case of Garcia v. The Minister of Citizenship And Immigration , a Federal Court judge stated:
 Moreover, the requirement to attribute truthfulness to an applicant’s sworn statement, asfirst enunciated in Maldonado v Canada (Minister of Employment and Immigration),  2FC 302, 31 NR 34 (FCA), reflects a policy that exigent circumstances facing fleeing refugeesmay compromise their ability to present corroborative documentation. Conversely, when aclaimant has, or may readily obtain, corroborative evidence in situations where it normally would be filed with the adjudicative tribunal to bolster the weight of an otherwise bareallegation, it is expected that the party will adhere to the ordinary reliability requirements tointroduce the best evidence in support of their case. If they fail to do so, less weight (or none atall) may be attributed to the statement.
In other words, if it is expected that you may have proof of contacting your wife (e.g. phone bills) then you have to produce those phone bills to support your sworn assertion that you have regular contact with your wife – just swearing in a document, or testifying on the stand that you contact your wife every day may not be enough.
The immigration department provides document lists for the various visa applications to Canada. With the exceptions of the document lists for Canada Experience Class and the Federal Skilled Worker program, most of the list of required documents are minimal and vague. Given that vagueness, it is important to consult with a professional such as an experienced immigration lawyer to determine what other supporting documents you may need to prove you qualify for a visa, over and above the basic document list on Canada immigration’s www.cic.gc.ca website.