I would assume you'd want monster's difficulty level to be based on the world difficulty. Beginner/Easy level, they're pretty passive and you can just pick them off. Normal level, monsters will co-ordinate to defend their lands and launch small, obnoxious forays into your territory to destroy outposts. Hard/Flee-For-Your-Life Difficulty, the ravenous beasts will throw everything they have to take you out of the game. But maybe I'm misunderstanding the nature of the question. If you could only pick one of the above three, I'd suggest the normal level difficulty. That way, the monsters provide a presence
I also assume that we're talking about the strategic map, not the tactical one.
I would prefer if their type or intelligence was referenced in their decision to attack.
I read this as something different, and kind of cool. What if the level of aggression were based on the monsters in question? Say you've got a spider swarm. They're mindless, they'll just wander around and attack anything that moves. Then you've got wolves. They wander around and attack anything the pack thinks it can beat. Thieves and brigands eagerly attack trade caravans, unguarded cities, and heroes they think they can overwhelm, but are dreadfully fearful of sizable armies. Maybe you remember the Haunter - perhaps a group led by the Haunter would only attack groups with a magically potent character (or some other cool lore reason). So basic motivations would decide when and where each kind of monster category would attack.