My best man, happened to be my best friend at the time. The person that out of all my friends, meant the most to me, that I felt knew me the best, and would throw me the best bachelor party (ok, I made up the last part, I had no bachelor party
)
But yes, it could be your brother, friend, etc... but I think it should be the person you feel has been through the hard times and the good times with you, that deserves to be part of the next chapter of your life.
Does that make sense? With that being said, sometimes it's luck of the draw. If you are blessed to have several close friends, that are considered of equal "value" as friends, then as the groom, I guess you would have to flip a coin or draw straws or something...