Well, if you accept that every level has equal chance to drop the items and you want to also get as many exp per energy, the ones to farm would be:
Name • Provinces with special drop • Ideal level
Battle Manual 15/19 15-9
Chainmail Shirt 16/20/23 16-9
Scabbard 17/21 17-1
Tall Boots 14/18/22 14-9
Arcane Scripts 12/17 12-9
Dagger 11/18 11-9
Leather Armor 9/19 9-1
Sharpening Stone 10/13/20 13-6
Strong Hope 12/21 12-9
Wooden Shield 8/22 8-7
Adventurer’s Kit 5 5-8
Practice Sword 4 4-6
Rugged Clothes 7 7-7
Training Manual 6 6-8
Dragon Bone 6 6-8
Meteor Fragments 7 7-7
Orichalcum Nugget 5 5-8
Fine Steel 19 19-9
Grimoire Dust 20/23 20-4
Hardwood Lumber 21 21-9
Midnight Roots 22 22-9
Crypt Mushroom 11 11-9
Crystal Shard 13 13-6
Firestone 14/23 14-9
Potent Leaves 16 16-9
Sunspire Feathers 18 18-9
Clean Cloth 5/13 5-8
Comom Herbs 1/8 8-7
Crude Ore 6/14 6-8
Large Bone 2/9 9-1
Leather Strips 3/10 3-6
Oil 4/12 4-6
String 7/15 7-7
Some of it are 11 monster and 9-13 monster levels, not 11-17 ones. But that because I have no proof that more monster will drop more, so I preferred focusing on energy and EXP as secundary priorities.
It’s really hard to figure out how the game rolls for items because I don’t know if repeated items were rolled at same time as one or two separated ones. It’s also hard to figure it out because the average for all of them is around 11-12 monsters, getting 17 is an almost impossible event, that would mean 3 consecutive waves of 5 enemies, plus two bosses.
Also, I kinda assumed a few lvls, so this is more an hypothesis kinda of thing.