I have seen quite a few times that the accepted answer is not always the best among all the replies (obviously under my own criteria). If I'm not wrong, only who asked the question is the one who can accept an answer... But, don't you think that the one asking is not necessary the one with the enough knowledge to accept an answer?
Not so much.
The accepted answer only means that the asker has taken that answer as the best for them. This is the privilege afford to the asker.
So, by definition, the accepted answer is the best answer for the asker.