The best one I've found, and I think was mentioned earlier in the thread, is the first battle of the Starter Dungeon. Yes, it's a 10K gold expenditure in the Shop to open the dungeon, but you make that up in just one round. Each time you fight the battle gets you between 10 and 50K gold, and you can do it up to 50 times per day.
I'm still hoarding gold like a crazy person, though. Gotta get those level 35-in-a-week-castle prizes.![]()