Now after 2nd game i have a better overview on units and so my favourites :
As for dragon emerald definetly
As for units for mid-late game :
Emerald again
Skell archer
Demon
Royal snake
Hunter
Sprite
Like fairy
Shaman
Inquisitor
For early game i like mostly
Royal snake
Inqi (if avalaible)
griphon
archmage
cannoner
ancient/polar bear