To me, best friends are those who will not judge you even if you are at a naught with them. They will, after voicing out his/her opinions, in the end respect the choice you've made. They will help you out in anyway possible as long as it do not go against their principle. If they do, they will refuse you frankly and politely. If you've made the wrong choice, they will simply accept that it's just a bad choice and then help you out in choosing the right ones after that. If you've made a mistake, they will learn to accept that it's a mistake and help you right what went wrong
I've made a vow.
If and when I have children, my wife and I will try our best to be their best friend and do all the above, so that we will be one they will turn to for advice, for help, for them to share their joys and sorrows and whom they would rather seek to share their secrets.
Because at the end of the day, if your children decide to put you aside in things that concerns their emotional well being, then you're no better than a stranger who only gives them shelter, food, clothing and money. When they can find it elsewhere, they'll leave in an instance, especially if it comes with more than just material provisions. That is when your children starts to avoid you, rebel against you, hates you and make parents' worse nightmares come true.
Like Mastercard's ads say - "There are certain things money can't buy".
And money can't buy a true best friend.