In whatever place you are in, in whatever business you are in, if you think you are not the best then you are in the wrong place my friend. The best thing to do is to leave that place and go to a place where you think you are the best.
Let's be realistic here, this is a competitive world, this is a nasty place. If you don't want to get replaced, if you don't want to lose, if you don't want to lose your job then your mindset should be... you're the best.
Let's take boxing or MMA for an example, if you think you are not the best fighter in the world then you will lose in your next match for sure and that is guaranteed. The moment you step on that octagon or ring, you should think that you are better than your opponent or else he will dominate you, he will shit on your face, do you like that?
Not only in sports but in any field that you are in, whether you are in the office or school or in business, your mindset you should be you are the best if you want to get on top. Because if you have this kind of thinking, success will be attracted to you. You will also do a lot of work in order to become really the best.
If you think you are not the best then you will be easily defeated. You will lower your standards and your competitive nature will be so low. You will settle in mediocrity, it will be ok for you not to win.
If you can say that you are the best in public, say it, make sure that you can back it up. but if you are shy to do it, just keep quiet but say it to your mind "I am the best" repeat it over and over again until it becomes a reality.
There is nothing wrong in thinking or saying that you are the best for as long as you are willing to back it up. Don't say it for the sake of just saying it and being arrogant or impressing people. Say it because you want to be it and you want to train your subconscious mind to become really the best.
Say it because you really believe in it and not to just intimidate opponents, say it because you are working hard to become the best and you know you deserve to be the best.