I'm against it.
For something like Yahoo it's an ideal system because it revolves mainly around straightforward questions. Someone just wants to get a reply on a question he or she has and then it stops. Other people who also want to have an answer to that same question, will end up on the thread with the first person who asked that same question when they search for it. Consequently having the best answer at the top is very convenient as you're only searching for that answer.
For example you want to know in which year Charlemagne was crowned emperor and you see that someone already asked that question on Yahoo with someone saying the year 800 tagged as best answer at the top. Well you have your answer and you're gone again as you're satisfied. You are not interested in seeing three people arguing endlessly whether it's now the year 799, 800 or 801 and you want even less to participate in it. The only thing you want is a clear answer, nothing more.
This is fundamentally different from NB as here the point is to have discussions. Even threads of people who ask clear questions inherently have most of the time the character of being discussions as people can contest/disagree/add to earlier given answers. Having a 'best answer' option is not feasible for NB as there is no such thing as a 'best answer' and in fact it even works counterproductive. When people hold discussions, you get a chronological order of people making posts and normally most people read all the posts that already have been made in that thread or in the least some of them. They base the contents of their posts on that of other people's posts and even first have a series of interactions with those people before making a 'final' answer, which is then again followed up by interactions with other people who argue with that answer.
With other words if such a post then ends up at the top supposedly for being the 'best' answer, it gives the impression that that answer is apparently the truth. Firstly this goes against the concept of a discussion where there is rarely something that is the real 'truth' and secondly it also gives the impression that that answer was created immediately and completely after just reading the first post, while in reality it was the result of multiple people, directly or indirectly, interacting with each other. Each post makes the most sense in the place in the thread where it was originally posted. If it gets removed from there, it gets ripped out of its original context and placed in another, incorrect one. This way the post loses its integrity and can cause misunderstandings.
And then you of course have the issue of people trying to get not the best answer, but simply the answer they want to have at the top of the thread.
So yeah I'm against it.