AB used to have a 'star rating' system for answers but it relied upon a system which could give rise to browser security concerns. (At least, I believe that's why it no longer exists. It disappeared around the same time as we lost the ability to use bold or italic text).
We currently have 'Mark as best answer' available for questioners to use but, somewhat irritatingly, it doesn't currently do anything on individual threads. However it does permit the Editor to compile a 'league table' of the top five people in the rankings.
Chris