although Liches and Witches are also good)))
dwarves like the Wall spell...
Overlords are good because of Rain
P.S. units that have only one spell (like a Druid with Swarm, etc.) are not casters... IMHO... because there's only one spell, no choice... so it's more of an ability than a spell))) at least, such units definitely differ from ordinary casters... at least because they can also fight in melee without being much different from other creatures of the same level).