Instead of pinning the accepted answer unconditionally ar the top it will be better to just add certain bonus number of points (e.g. 10/15/20) to the answer and then order answers by points number normally. The actual number of added points can be shown next to the tick. The number of points can be different for different sites(for small sites the number may be less).
Update:the similar idea was mentioned in Modified answer view based only on votes, not accepted status