Seriously, when I call someone my best mate or bestie as we normally put it, it means that such individual holds a very dear position in my life. It's someone that I would be willing to do anything for, someone that I would be willing to go any mile for in order to make the person happy because I'm well aware that he or she would do exactly the same thing or more for me as well.
It's not everyday that you get a great friend who would mean even more to you than the way your immediate family did mean to you. Such a friend is already a friend turned blood brother or a sister because he or she would always be there for you in both good and bad times.
There are some people who pretend to be your friends but they are just parasites to your life and would never add any value to your life.