The most useful monster.

Cyclops, Djinn

Wow, such an attitude that you just want to argue:D
Cyclops, in my opinion, is not useful, especially against Haven, because forgetfulness affects it, and that's it, the cyclops is useless.
The only plus of the cyclops is to shoot it so that it hits the hero, but there are also options for that:)
And Djinns... when they are against you, you regret that there isn't a spell that would destroy the target's mana:D