As an expert in the field of Guinness World Records, I can tell you that the most common adjective associated with the Guinness Book is ‘FIRST.’ This is because the Guinness World Records is a compilation of all kinds of records, and the vast majority of these records relate to someone or something that was the first to accomplish or achieve something.
When something is deemed the ‘first‘ to accomplish something, it means that it was the initial instance of something happening, and that no one has accomplished it in quite that manner before. It’s important to note that the ‘first‘ doesn’t always mean the best or the most successful, but simply the first of its kind.
Many of the records in the Guinness Book would use ‘first‘ in their description. For example, the first person to swim across the English Channel or the first dog to climb Mount Everest. These types of records may become outdated or broken, but they will always be remembered as the ‘first.’
So, when searching for an adjective to describe the Guinness Book, ‘first‘ is likely the most fitting choice due to the overwhelming number of records that revolve around the concept of being the very first to accomplish something.