Orc Veteran is the best !
 

Did you notice they attack have 3 attacks every round ?
1st attacking
2nd attack as an answer to the enemy's counter strike
3rd normal counter strike.

take 200 of those and they can deal 15,000+ in the first round easily.

Does anyone know of any other unit with this ability ?

demons and executioniers and grphons always retaliate not just 2 times. also black knight with shard of darkness
IN legend i was doing 100k+ damge per round with demons.

They are not retaliating on the counter strike of the enemy, although they have multiple counter strikes - it is not the same.
Im talking about attack-get damage from defence- and attack again

This ability is indeed nice, but they are not really fit well to a no loss / impossible group.
For a tank unlimited retailation is just better.

No -- that is the only unit that currently has that particular ability. I personally don't find it to be all that exciting in practice.

The only similar ability I can think of at the moment is the Brontor's armor, which will (assuming responsiveness) fire off on an enemy's counter-attack when the Brontor attacks, but then of course the damage is not comparable.

What about sticking a Mana Shield on those furious guys and sending them in? Get +15 D, and +15 mana. More, if it's against Undead or a Demon. Enough mana to cast Resurrection and get them back. A pretty good deal all around if you ask me.

Brontor's kinda work like this too. And they can hit three stacks at once.

Also, Orc veterans still require that the opponent gets to retaliate. A red dragon flaming a row of 3 guys doesn't have to worry about that :).

I guess Orc Veteran is good for clearing summons in boss fight.