3 ideas
Each ad variant carries a coupon or return code unique to it, so every inquiry or sale can be traced to the exact headline, copy, or medium that produced it. Small test campaigns run side by side, and only the variant with the lowest cost per customer is scaled nationally. Opinion, taste, and cleverness are replaced by counted returns as the arbiter of what works.
Generalities like 'best in the world' are discounted by readers as expected puffery and leave no impression. A precise, factual statement, such as a number or a concrete process, reads as true and considered, so it is believed and carries weight.
An ad should be judged exactly as a salesperson would be: by what it costs relative to the sales it brings in, not by how entertaining or admired it is. This means copy should address one individual's self-interest, offer service instead of asking for a purchase, and never stop at mere attention or brand-name repetition. Fine writing that doesn't sell is a cost, not an achievement.