The most USELESS monster.

That's not entirely correct. There are indeed useless monsters. They can simply stand there and hinder the movement of other units. (Will a single fairy on the battlefield actually do anything!?) It needs to fly to the archer in 2 turns. Before that, the enemy units move forward and simply kill the fairy along the way. Who did it benefit? No one, it just occupied one stack. The same applies to a large number of fairies (30-40).

But what am I talking about?! You, for example, took these fairies into battle for some reason. If they were just getting in the way, why did you take them?

So, there's no need to say that all monsters are useful; that's obvious. We're talking about something else here.

I wouldn't say "all are useful" or "all are useless," but rather I would talk about the advantage of one unit over another. There are situations where pirates are more advantageous than skeletons, and leprechauns and satyrs are more advantageous than wolves or tigers, for example.
With this statement, it is impossible to definitively determine a monster that is ALWAYS USELESS. That doesn't exist!