I used to work for a VP of Sales & Marketing. He always wanted us to put in the print ads that we had: "the best, the finest, the number one, etc." he had the attitude that others had to prove that we weren't. And if the FTC or FCC (for broadcast) told us to quit making unsubstantiated claims, we'd pull the ads and find other words to say our products were the "best." No one ever challenged our claims. Back then, the FTC and FCC had rules in place for claims and sales. I don't know if there is any oversight over online advertising other than the host's guidelines.
Some people like to deal with the best, the number 1, the top, the most popular, largest, fastest growing, etc. They feel the seller is Number 1 for a reason. Others like using them because they feel there is safety in numbers.
I usually avoid places that claim to be number one because quite frequently that means they have the most aggressive, high-pressure sales people on staff.
Rhiggie is right, you can find a metric to support most anything. MY favorite are the "JD Power Awards". They seem to give one for every trivial thing you can imagine. Kind of like the old "Who's Who in America" books. You paid to be listed in them.