At 1/2 energy cost, four fold pillar in st lamia's final dungeon series is the best if i recall correctly. 5 energy for 2600 points (520 exp to energy ratio). After that, you are looking at end game maps in Mirvana being the divine emperor at 23 energy to 7,100 exp (308.7 energy to exp ratio).
Time investment is going to favor four fold pillar as well. I think i did a breakdown on this before where you would only have about an hour or so downtime at worst up through the 80s in summoner levels. Post level 120 it gets worse though. If we are also at 1 energy per 3 minutes, that pretty much flies out the window for near unlimited uptime for grinding since it takes a fair amount of time to auto battle any of the maps. I haven't checked if the game updated that or not. I could be reading a comment out of context too.
edit: spelling
Time investment is going to favor four fold pillar as well. I think i did a breakdown on this before where you would only have about an hour or so downtime at worst up through the 80s in summoner levels. Post level 120 it gets worse though. If we are also at 1 energy per 3 minutes, that pretty much flies out the window for near unlimited uptime for grinding since it takes a fair amount of time to auto battle any of the maps. I haven't checked if the game updated that or not. I could be reading a comment out of context too.
edit: spelling