Lot of good answers. Boris and Steadfast are also good at farming Star Coins. Of course Mary is the best with her x2 xp skill. In the case of a Steersman, take his 8500 XP and multiply by 2.3 (Marys attribute plus Fast Learner). And that doesn't include any supporting troops in the camp.
I need to give credit where credit is due: Blamuut is responsible for getting me to realize how valuable Mary is as a Star Coin farmer.