41-60 Old Haven, kill skeletons and zombies
60-70 Lizardmen (Despise, Rikktor spawns, Tokuno champ)
70-80 Earth elementals (Too many places to mention)
80-100 Troglodytes, elementals, fan dancers, etc
100+ Yomotsu warriors, minotaur, Ogre Lords, Arctic ogre lords, etc
That's my chosen path. Others have different paths. There are many to choose from.
I know one guy who did demons all the way from 50 skill to 100 before he went to ogre lords from 100 to 120. He called for a lot of ressurects early on in the process.