In highly competitive product categories, the race is always on to claim "the best". And product manufacturers who make a great product and who can substantiate "best" claims should be able to make them loudly and proudly.
In a recent NAD case, T-Mobile was found to have taken the proactive but needed step of discontinuing its "America's Largest 4G Network" claim. It could make this claim and had made this claim for some time but the size of competing networks grew. T-Mobile nimbly reported it was focusing in a new campaign on the speed and dependability of its network. But not soon enough to avoid the NAD challenge by AT&T.
The problem is the burden of making such a claim is you need to continually watch it. An unqualified "best" or "largest" or "highest" (or maybe any "-est") claim the NAD repeatedly finds means you are trumpeting superiority compared to all or at least a significant majority of products in the class. The requirements are not hard and fast but a good rule of thumb is testing against 90 percent of the product category based on sales (likely unit rather than dollar sales). Of course sometimes a best claim can be a puff (e.g. "Best Buy") but it is always a risk to assume your claim is not a claim at all.
Even when you get past this high hurdle, the claim needs to be substantiated on an ongoing basis. If there are new products introduced into the product class or exisiting products are improved, these need to be tested. And if your "best" claim appears on product, and somebody builds a better mousetrap, you may be confronted with revising labeling and packaging off schedule. Of course sometimes a best claim can be a puff (e.g. "Best Buy") but it is always a risk to assume your claim is not a claim at all.
Another example of the need to monitor is a recent case where NAD recommended Church and Dwight discontinue its claim that it's First Response digital ovulation test is the "First and Only" test to predict ovulation using an adaptive algorithm based on a woman's individual hormone level. A competing maker of Clearblue Easy said it used a similar mechanism, even though it had not advertised this fact. NAD noted that a "first and only" claim necessitates the advertiser having some information about how competing products operate, which may or may not be readily available, so while it did not fault Church and Dwight for making the claim originally, it recommended discontinuing it going forward.
So before committing to a "best" or "only" claim, make sure you have resources in place to continue to monitor the accuracy of the claim. Your competitors certainly will and like AT&T and others know where to find NAD at their new address to bring a challenge.