What do the game devs consider the best units?

[Gatts]

Griffins:
1. easy to replenish (ressurection, sacrifice works fine + divine armor, armor of the gods etc. - you can buy zounds of them - store them in the spare back up slot and re-fill if you play impossible - still you won't lose any battle)
2. endless retaliation - enemy mostly have many units - more than 5, more than 10, returning attack to most of them gives your unit additional hidden damage ratio (like you would have 5x griffins on the battlefield)
3. split - if you fight many but weak stacks - you may finish them much faster
4. flying
5. natural resistance (not as good as of cyclops, but still good enough)

Vamps:
1. steal life - I never fought army (other than pure undead) that didn't have at least one "living" stack.
2. flying
3. cheap and plenty of them to up-hire in back up stack (for battles with undeads - where the steal life is not working)
4. mind immune
5. even if ressurection does not work, still works sacrifice - if you don't have enough money, but you have sacrifice spell - use it on vamps to replenish other vamp stack - the one that lost units can gain them from blood sucking.

For both of these units is very good to know:
gain additional XP by using only these units + armageddon - your XP will grow because you would be "weaker" compared to the neutral stack, but using god armor on griffins + ressurection, or using steal life of vamps - will make you the victor with minimum loses, even if your enemy was ranked "impossible", just imagine how it is to get 11000 xp instead 500 ... each time...