https://www.graalians.com/forums/showthread.php?t=35683
Aside from rares (which are actually rare now), I would say that lizardons are the best if you know how to kill them quickly (No, not waiting for fire breath, though that is convenient) and you're not looking to farm bombs for profit. You get a large drop of gralats/arrows/bombs with them, which is fairly randomized, meaning you get a decent amount of cash in general.
If you're looking to make bank killing baddies AND farming, try the pirates. You can get far more pyrat kills in a short time than lizardons, leading to bombs and arrows (seemingly less cash drops with these guys, and the odd map). Rebels are roughly the same, but don't have like 10 npcs in one level like the pyrats do.
If you're looking to do the maps AND farm, then bandits it is.
But looting is better than all of these, if you know how.