The most efficient way to populate the Region field on the Lead based on the zip code is to use a Map to store the Region__c records keyed by the Zip_Code__c field. This way, the developer can avoid querying the Region__c object for each Lead record, which would cause a SOQL limit exception. Instead, the developer can query the Region__c object once and store the results in the Map. Then, the developer can loop through the Lead records and use the Map to get the corresponding Region__c record based on the Zip_Code__c field. This approach reduces the number of SOQL queries and improves the performance of the trigger.