If you have to say you are, you’re probably not.
“The best pizza in the world!’
“#1 in quality”
“We do our best for you.”
If you had to do one or the other, you are better off doing than saying. The problem with most businesses is that they come up with a self-righteous slogan before their doors even open. You are better off using a slogan like this as a private, self-motivation tool rather than a public, self-proclaiming tool. You should try your best and let the world decide if you’re the best.