We've found that RCI generally has the best fares for one way or open jaw flights. When we book a cruise, we also book airfare with RCI as we can cancel it anytime before final payment. We know what RCI is charging and keep shopping for better fares until final payment. If we find a better one, we book it and cancel with RCI.
We can speak only for RCI as we have not done any repo cruises with any other line.