Also the literally BEST units to take over are those:
1. "The Dark Sorcerer" from the Quest where you rescue "Bill" (a stone golem)
His qualities: Level 20, has "Despair", "Summon a level 20 evil thingy with good attack" and "Cloak of Fear"
Pretty overpowered because he is just a unit and his casts do not cost mana. Also Despair pretty much singlehandidly wins fights (keep in mind it does 3xsorcerers level as damage to everyone and even if resisted still does some damage meaning that if he gains any levels he will become even stronger). [The Dark Sorcerer is the strongest version of this kind of unit ingame, at least to my knowledge, but you could try to "tame" any of the fully upgraded graveyard units, the lich, the sorcerer there or maybe the banshee (although i don't know if shes twisted)]
2. Ongr the Unbound (he used to be the best target back in the days when you could upgrade his "group" to have 6 of him 
. But since that is "fixed" he is only 2nd here). Equipped with the "Scythe of the Void" and the "Maul" ability he isn't to be underestimated. If you can grant him any form of +%exp he can even levelup slowly and potentially become stronger.
3. The Ogre with Curgens Maul from the Master Quest (he is "Epic"). You still recieve Curgens Maul because you "won" this fight so you have that Ogre and another hero with that Weapon (if i remember correctly it has approximately 60 atk).
4. Well Ascian herself. Her skillset is pretty avarage for an assassin, but she does have Maul. Nothing else really good here though as she counts as a hero and WILL sap experience.
[5. Maybe from the Quest DLC there is the other Dark Sorcerer (unsure if twisted, didn't try because the quest is very rare), he drops this 20 armor/60spellresistance/60Spellmastery robe which is pretty good. I don't remember his skills, but I guess he would be good too.]
6. Potentially any strong darkling shaman now that since version 1.4 they can summon other darklings.
7. Troll (Shamans) aren't too bad either, but nothing special here.
