It's in the title.
What I do is picked the answer that most clearly and concisely answered the question, or I pick the answer that was most well-received. If everything is just an opinion, I roll virtual dice and assign a number from the dice to every answer that actually addressed the question.
Most Helpful Girl
I pick answers that fully address the question(s), ones that use reason and are thoughtful, clear, helpful, unique, informative, and well-written. If I'm asking a trivial question like, "on a scale from 1-10 how well can you dance?", then I'll pick an answer that mentioned something interesting (relevant to the question), or an answer that made me laugh.
I always give best answer, because I know people put time and work into what they write, and I want to make it worthwhile for them and acknowlege answers that stand out to me. People feel good about getting best answer and are encouraged by it, so that's why I do it. I give everyone who answers a +1 to show that I appreciate their answer, although, back when that meant an extra point it probably meant more.3