That statement is extremely subjective. Some people are indeed the best or biggest at whatever they do in life and there is no one who is bigger or better.
Not to mention, that at a certain point everyone is in the "best" category (or the highest percentile). In fact, are lot of people on this board are in the highest percentile for cock size. An inch here or a half inch there becomes irrelevant at that point. Sports are another good example. There are a lot of great football, basketball, soccer, etc. players. It's hard to say who is better between Kobe Bryant and Lebron James. In essence, they are both in the category of "best at what they do".
I think it's important for people to be able to say they are "one of the best" at something in their lifetime, especially if it's something they put a lot of time, energy, and effort into. Could be something simple, could be something complex. Could be playing a video game, could be scientific research, could be the best worker at your job.
But thinking and saying, "well there's always someone who is going to be better" is probably going to lead to more insecurities than striving to be the best at something even if you never attain that position.