You might consider doing it LinkedIn style, where the poster rates answers as "good" and (optionally) one as "best". The answers are displayed best - good - others. It would be a matter of sorting first on "goodness" and then on date. Say, "best" is 2, "good" is 1, default/others is 0.
This would normally be combined with something that LOCKS (Closes, in LinkedIn) the topic so no more answers can be added after good/best have been selected. A PM or email could be sent to all whose post was selected good or best, congratulating them on that. Some count could be kept of good and best answers, to be displayed in the info area as a badge.