The most useful monster.

On the contrary, I need everyone in my army... I try to build all creatures so that I can make the most of the race's advantages.

You can only build everyone if you have several Castles. Meaning, are you suggesting building one thing in one castle and another in a different one??? If you don't have a clear advantage in everything else, then it's a guaranteed loss.
And you can compare the usefulness of units; for example, a skeleton and an imp:
their strength per growth is roughly the same and small, but the imp travels far and gathers resources better; even in small numbers, they can cause significant trouble in battle by draining 1 mana from angels and cutting off their resurrection, and in large numbers with tactics... As for the skeleton—it's just a product of early Necromancer magic and mediocre fodder (although it's great for Sacrifice, but that's more exotic)/
And then there are units key to development, like vamps and djinns.