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The question in the topic is indeed not quite correct, because, naturally, Champions of Darkness are the strongest after Cyclops Dukes in terms of HP and damage, and of course, also in terms of protection, plus magic resistance and breath attack.
However, if we consider their usefulness, it's very difficult to say, because you can't apply buffs to Champions of Darkness or heal them, while Cyclops Dukes, with Berserk, Hypnosis, or even a simple counterattack when built in a square formation, can wipe out all your buffers and other monsters (well, maybe almost all).
Wizards are good with a Summoner – you divide them into squads and confuse neutral creatures, while you summon Phoenixes or Manticores for yourself. But if you have Dragon Strength at level 5 magic, then in all castles, Cyclops Dukes will be much better than Phoenixes, and Manticores are fun, they stun well, and I'm not sure, but if you can't cast spells after their stun, then that's really great, and they also fly well, although there aren't many of them.
Champions of Darkness and Hydras – here, Champions of Darkness are probably better, even without buffs and healing, but they fly far. However, Hydras, with Catapult and Assassin, will easily take out level 4 creatures; even Angels will fly in, attack once, and then they're gone.
Angels vs. Champions – more likely Angels, because resurrecting heroes is a tricky business, and they can also resurrect themselves. There are simply more Champions, and often when you raid a Griffin Nest, it's much easier to buy them. I, for example, take Champions because I'm going for Crusaders, not Monks; I'm just more used to it, I like it more, etc. You can buy Champions almost immediately; you just need a stable, but if you judge objectively, Angels are probably better, even if you use both skillfully.
Titan Golems – no question, Titans, because you can't heal Golems, and you'll be losing them to neutral creatures. I'm not even sure if you can raise them with Resurrection.
Devils vs. Bone Dragons – it's complicated. If you're fighting against Order, you need to take Bone Dragons because they have twice the defense against Titans (it's difficult to hit Bone Dragons with arrows). Devils are very useful if they run away to the other end of the map and survive, although after being hit by a Catapult, they don't have a counterattack. But if your Demons manage to freeze a buffer or an Angel, that will be more useful.
Monsters vs. Birds – I can't judge about Barbarians; I only played the campaign, and I don't know how to play them, and I don't really want to learn; it seems too difficult to me.
Dark Champions – they shouldn't have been left out; they are very useful. I would choose them even with Champions of Darkness present.
There's no point in discussing Sea Monsters because you can't play with them, although maybe you can with Diplomacy