Return to Ruins should be your main hangout for much of your unit XP farming. It has the best non-specific unit XP bar none (24.13 XP/C), and with dropped units being level 14 it has good gold efficiency.
If you want to focus on leveling your Prince, Immortal Beast and Dragon Hunting are best. (Tower Overwatch if you haven't cleared Castle Retake yet) They're also good if you don't have a lot of real-life time to burn as you'll go through CHA quickly. You'll level units extremely slowly with these, but you'll be making lots of extra gold as a result, so they're also a somewhat decent pick if you need to farm silvers from 2k shrine/gacha.
If you are leveling a Mage, Flame Dragon is probably your strongest option for max efficiency. Its only 20.6 XP/C for mages, but you will be saving loads of gold while doing it.
If you are leveling a Soldier, Remnant Vanquish 3 is a really solid choice. Same principle as Flame Dragon; slightly lower unit XP than RtR, but waaaaay better gold efficiency than RtR. Its also just a really good overall map for many other reasons (fast, easy, good CHA efficiency, etc.)
If you are leveling a Heavy or Archer, I would personally stick with Return to Ruins.
Tower Overwatch for Heavies and The Wolfpack for Archers are decent alternatives if you're bored of RtR. Slightly more prince XP and gold, but slightly less unit XP.
Gold = Deep Forest Road. Sell everything. Roll 2k gacha. 'nuff said.
Why Deep Forest Road and not Pastoral Gate? Because all 5 unit drops are not 100% guaranteed. If they were, Pastoral would win out. But Deep Forest Road has a whopping 112.5 Gold per CHA not factoring in unit drops (vs 75 G/CHA from Pastoral). If you are using the 3 crystal gold bonus, Deep Forest Road pulls even further ahead at 203.8 G/CHA. DFR is also a shorter map, time-wise (CHA/minutes).
If you're to the point where you can farm Immortal Beast, that means you probably don't need this sheet. But its actually a really good overall level. Maybe even the best pick if you're not focusing on any one specific thing. Lots of G/CHA (107.1!!), and unit drops are LV20.
A very specific situation: if you are REALLY REALLY REALLY desperate to level an Archer and Soldier RIGHT NOW, and specifically at the same time... Pastoral Gate has a whopping 29.17 unit XP/CHA. But the soldier/archer drops are only level 3, so you are going to be burning your gold at an alarming rate. You might also go nuts since Pastoral only costs 12 CHA, meaning you're going to be repeatedly staring at that map for at least several hours every day. Basically, only farm Pastoral gate if you're a super rich premium player with no need for silvers from 2k gacha and need to turbo-level archers and soldiers at the same time.
Gold = Deep Forest Road
Unit XP = Return to Ruins
Lifeless Slug = Pastoral Gate