Take this question for example. There is no accepted answer because I was looking for other ways to script HTTP more efficiently than using Java/apache commmon's HTTP Client. How come I have to choose one answer, yet there isn't even a right answer to that question?
closed as too localized by GEOCHET, Ladybug Killer, juan, alex, Ether Feb 2 '10 at 17:07
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No one's forcing you to accept an answer and no one's forcing you to have 100% acceptance rate. Just leave it the way it is, most of us understand that some CW questions simply can't have a best answer.
How come I have to choose one answer, yet there isn't even a right answer to that question?
You don't have to do anything. If you don't want to, don't accept an answer.
Recognize, however, that you had a very broad and open ended question to start off with. You wrote a question which, by definition, does not have a single universal answer. So you already chose the path that leads to no answer.
However, you should at least vote up everyone that contributed something that is useful - in this way you not only highlight their contribution as a reasonable contribution, but it's also a 'tip' of sorts to give them reputation for reading and answering your question.
You might also consider objectively reviewing all the answers, and selecting the one where the contribution was above and beyond what you originally asked. It may not be the answer for your question, but you are simply recognizing that they provided the best information given your limited input. Post a comment on their answer indicating that while this isn't the answer, you appreciate their time and selected their answer because they obviously put significant effort in it and you want to give them the additional 15 rep tip for their help.
While the site has suggestions for what you should upvote and select as your answer, at the end of the day they mean whatever they want to you. They highlight the answers in a certain way, change the ordering, and give reputation to the contributor. If you feel an answer is deserving of those features, then you can provide them even if they don't meet the site's definition of 'answer' or 'upvote'.