All rightie. After a week of thoroughly experimenting I am ready to bring to the table my own conclusions regarding farming
1. Dragon Hunting is definetly the best one for char levelling. Captain Obvious much, i do understand
2. Less obvious is that Remnants Vanquish 3 is the best map to farm fodder for levelling your units as long as you are ugrading non-archer and non-mage units.
This map has the highest avg overall drop rate (3.5 with a minimum of 2 iron unit drops guaranteed), as well as the highest average copper unit drop rate (0.87 per run).
Now, with bonus exp for matching unit types (soldier here, which also relates to katie, julian, aria, etc.) and with bonus exp for copper units, each run from this map brings in an impressive +12% to +19% ROI compared to any other high level map. Again, that's as long as we are talking about non-mage and non-archer units (so valks, HA, witches, soldiers, bandits, etc.)
Remnants Vanquish 3 also does not require micromanagement, is quick to complete (so you don't have to spend hours farming it), and is only a tiny bit worse for lvling your main char as compared to dragon hunting.
3. Another CO bit is that flame dragon map is obviously best for lvling ur mages.
I'm still trying to figure out which map is best for archers.
On the side note, why is Return to Ruins so popular for farming with many players? It does lag behind against two specialized maps at least (remants vanquish 3 as noted above, and flame dragon).