Honesty stands very high on my list.
It is not what he is to the world that you should look at.
It is what he is to you that is far more important.
He should be someone whose name strikes you first when you are in a mess.
I think that the most valuable quality of a best friend is trust, complete trust, as simple as that. If you don't have it then that person really isn't a best friend at all! If you have that kind of trust, then the loyalty is already there.