The most useful monster.

By "benefit," I mean that the advantage remains even after death.

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'll already have a whole bouquet of debuffs (cowardice, weakness, sadness, curse, old age, misfortune, etc.). And for ranged attackers, there's also forgetfulness.
The only problem is that they aren't found in castles!