The most USELESS monster.

probably for me, Troglodytes
immunity to blindness only saves if there are a lot of Troglodytes in the army, and the hero only has blinding as a spell: D
Pirates - on the sea, they are more or less okay, if you compare their strength to other units, and not - who is stronger, 100 pirates or 1000 wolves: lol:
and what about Troglodytes?
when there are many, any unit is good
and when there are few, well, they are only good as cannon fodder
BUT! ANY UNIT IS GOOD AS CANNON FODDER, sorry for the caps

so the question of the most useless unit is purely rhetorical
because if you consider the specifics, then many units can be useless (if not all) - because it depends on the situation ;)