Best and worst units per level

[Vilk]

That depends not only of the available units for the context but also of the items available. For example with the Anga's ruby:
1: Lake Fairy
2: Dryad
4: Demoness


With Ogre Sandals and slot for it:
5: Giant

And so on. Well with no special items my favorites would be:
1: Lake Fairy & Skeleton Archer
2: Dryad & Scoffer Imp
3: Inquisitor & Griffin & Cursed Ghost (or are they level 4?) & Royal Snake
4: Ancient Vampire & Knight & Archmage & Shaman
5: Archdemon & Emerald Dragon & Red Dragon

EDIT: For a complete list of units there's gamebanshee. Some quote there some errors, myself I haven't quote any yet. At least the unit list seems complete.