The 'best answer' thingy has never worked. It replaced the 'star ratings' (with one, two or three stars) which questioners used to be able to award to answers, which seemed a good system to me.
I don't think it does anything at all. I posted a thread and marked one of the answers as "best", but all it did was remove those words from that particular answer. It didn't show up on the public view of the thread at all. I don't know what it's for.
Do we really need to be awarded or rewarded for answers.? I recall the issue having been previously raised and someone having mentioned the difficulty if a respondent had been given three stars and another ABer then came on the scene with a far better response.