I prefer the simple and more modest claims, primarily because it becomes easier to meet and hopefully exceed the expectations associated with this claim.

If you are the ‘worl’s best’ at something, then it becomes impossible to exceed the expectation associated with that claim – you can’t be better than the best. And being the best represents only living up to your claims, not exceeding them.

Claims using superlatives however, can be effective if you can back these up with figures. If your product wins some kind of official award or recognition for being the longest lasting or most durable or whatever, then you could certainly mention such an award – it is testament to quality.
Andrew´s last blog ..Niger Delta Crisis – Big Oil’s Big Lesson

I’d describe myself as a very good writer, but I’d never claim to be the “world’s best.” That sort of boasting would go against my brand of clear and straightforward copywriting — and would probably turn most clients off.
Rebecca Smith´s last blog ..Merriam-Webster’s 2009 Word of the Year: Admonish