The UK Advertising Standards Authority has recently shed its light on claims that rank a product. The claim in question was "Australia's No.1 Breakfast", used by an Australian breakfast cereal producer. Food information may not be misleading, meaning that such claims may only be used if they can be substantiated.
ASA considered that to substantiate the claim "Australia's No.1 Breakfast", proof should be provided that the product is actually the top selling product in Australia in terms of unit sales. Value sales was not considered sufficient.
What about products that are not top selling in terms of unit sales, but are for example the most discussed product in social media? The use of "No.1" claims on such products may be defended, but they need to be formulated carefully in order to not be misleading.