The most useful monster.

Then, in that sense, the champions are Beholders. Not only are they ranged attackers and have flight, but they also cast random spells. By the time some Squire reaches them, they already have a whole bouquet of debuffs (fear, weakness, sadness, curse, old age, misfortune, etc.). And for ranged attackers, there's also forgetfulness.
The only problem is that they aren't available in castles!