Best Unit to Sacrifice?
 

What is the best unit for sacrificing and building up your rare creatures? That is, best unit cost wise as I'm in impossible.

you should try to sacrifice without losing the units there are many ways to do it.
- Vampires and anceint vamps can restore their life back (before patch 1.7)
- sacrifice with low inteliigence + giants (who can be healed with shamans)
- double sacrifice + timeback
- sacrifice + hypnotize (or demoness seduction)

and finally you can resurrect your fallen troops with Inqusitors and spell.

After the 1.7 Patch, Undead and non-living creatures like Plants and Cyclopes can no longer be sacrificed (they can be gained by sacrificing another unit, though).

Timeback is a good way to cut your losses to zero with Sacrifice. Also Giants or Dragons are good candidates for Sacrifice.