Sometimes a question may not be very useful but the answer may be. I know that the accepted answer is the most useful one to the OP. But for other users, the most useful answer might be some other answer. Being able to mark that answer as a favorite answer would make it easier to find for future reference.
If you want a way of knowing what the best answer for you was you can favorite the question and then upvote the answer that was most helpful for you, even if it is not the accepted answer.
I see why you want a "favorite answer" option, but you can achieve nearly the same concept by just favoriting the question and upvote the most helpful answer(s) for you. This is the idea behind upvotes, just because one answer helped the OP and was accepted doesn't mean it was the best and that's why others can upvote the other answers to move them to the top for future users.