Setting the default value of the Lead Source field to Trade Show' is the best approach to improve data integrity for the Lead Source field. A default value is a value that is automatically populated in a field when a new record is created, unless the user enters a different value. Setting the default value to Trade Show' would save the sales reps time and ensure that most of the leads have the correct value for the Lead Source field.
Creating a validation rule requiring the Lead Source field to equal Trade Show' would not work, because it would prevent the user from entering any other value, such as Web, Phone, or Referral, which may not be accurate for all leads. Making an assignment rule named Trade Show' to only assign leads to sales reps would not work, because it would not affect the Lead Source field, and it would not allow the user to assign leads based on other criteria, such as territory, industry, or product.