There's a billboard in Chicago that millions of people see daily. It's on the entire side of a several story tall building, located at the on ramp for the interstate in and out of Chicago. It's for Jim Beam's bourbon. It's simple: on the right side side is a bottle of Jim Beam's. On the left, in large cream-colored letters against the dark background, it claims, "The best in the world." And near the phrase is a seal from the contest or convention (the light turned green before I could read it) that chose it as the best. Without the seal, the words would be mere conjecture. Someone, somewhere, picked up the glass, took a sip (or a swish, or a taste, or a swig), slammed the glass on the counter, splashing little droplets everywhere (classy whiskey drinkers everywhere shudder), and said,
"This is the best d*** whiskey in the world!"
But that wouldn't actually mean anything to anyone besides him. To make a statement that bold that means something, it must be backed up by more than solitary opinion. If you call your horse the fastest—it needs to have won gold in every race since it started running. If Grandma June 'makes the best chocolate cake in Missouri,' everyone in Missouri must agree. And when your clothing label is more durable than all the other leading brands, go ahead and tell people. But only if it's been tested to be absolutely true.
It's a quick way to lose your credibility—because everyone has "the best" cake recipe, the "most durable" clothing, the "fastest horse" (or car, or train, or internet speed). If you are quick to make claims about your product (animal, clothing, recipe) being better than everyone else's, no one automatically believes you. They are instead instantly on guard, thinking of their own amazing thing. It's not to say that yours isn't wonderful—but people are usually partial to their own.
Come back tomorrow to see why it matters in writing.