Pizza Hut once sued Papa John's for their slogan "Better ingredients, better pizza, Papa John's" as libelous. That they were asserting without evidence that Pizza Hut's ingredients were inferior.

Papa John's rebuttal was that as an advertising slogan, it was boastful and not factual in nature. That through their research, they only found one advertising slogan that was entirely fact-based, and that was Sony's slogan 'It's a Sony."

I know of at least one more, midwest fast food chain Rax had a slogan "You can eat here." which seems rather verifiable.