Most cost-efficient conjurable units?

[Azhur]

What do you consider to be best conjurable units in the game? By that I mean by taking into consideration following factors:

- Number of units when conjured
- Quality of the unit(s)
- Cost in gems
- Required magic paths
- Required research level in order to conjure

Thus far, I've only really been conjuring Iron Dragons and they are lategame stuff, so it would be nice to hear what are your favorites.