The most useful monster.

Personally, I find Chaos Devil to be the most useful unit, especially when playing as Chaos. 400 HP, flying ability, an attack range of two cells, and high attack/defense stats – that's quite something. By the way, I also voted for the Dark faction in Heroes of Might and Magic III and V.
Well, I just like them...
P.S. Pandora, you're a moderator now! Congratulations!