To get the most complete and accurate record in Salesforce, follow these tips when setting up automation rules. But you can get increasingly precise and control things down to the individual field values using Specific Field Values (Tab 2 on Automation Rules).
For example, if you choose to have the oldest created record as the master, you may want to have the newest modified Account Phone field always override the master. What this means is that if the phone number has been recently modified (most likely because it was updated) then that phone number will be used for the master record, raising the probability that you have the most up-to-date phone number. This same method can be done for addresses, emails, titles, etc.
For more information on setting up specific field rules, please visit our Merge Automation Rules help article.