Without rapid respawn go for the rares: the ones that only need 5 to be killed, as the elf overseer for example. And then, go for the ones of the places you want to hunt in, because remember that you can only use the charms in the creatures you have unlocked.
In rapid respawn first go for the ones that can die with a firebomb without you on the screen as rats, frogs, etc. Then, or on the mind time, go for the 250 ones which are easy to find or/and kill, and are several creatures in the same respawn (minotaurs of darashia for example)
The 2 links that ellotris guardian gave you, complete the informationn you need.