I don't see the need to downvote the question. You should upvote the answer that helped you the most. The accepted answer is the one that help the OP of the question, you can't really say what was good for him.
If the accepted is really wrong in some way, you could downvote and leave a comment saying why this particular answer is wrong.
But keep in mind that the accepted answer is only there to say that it help the owner of the question.
From How does accepting an answer work?
The bottom line is that you should accept the answer that you found to be the most helpful to you, personally.