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When I was just starting to play Heroes 5, I was a big fan of Chaos Devils. I always felt bad when I lost them, and I remember really hating Emerald Dragons. But now we can draw some conclusions (since part 6 has already been released, although it's a complete mess):
Archdevil: In principle, it's a decent unit, has a chance to reach the enemy in the first turn, under the command of demons, it feels like it hits much harder, summons Lords, but... something is missing, unfortunately...
Archangel: An interesting choice, but 50-50 damage completely negates the point of blessings. Also, such high damage significantly lowers its appeal in my eyes, because even the 2-8 damage of an archer gives a chance for a "maximum uber hit" if you're lucky. The only plus is speed (but what's the point?), plus resurrection (usually of paladins, and even then, it's a one-time thing). Not very good...
Ghost Dragon: I completely skip this one. At least give it high speed, but as it is, it seems like Messengers are more powerful. Besides, I usually build them quite late, because I usually don't have enough resources, so by the end of the game, they don't have time to gain strength "in numbers." Result: a failure...
Titan: Now this is more interesting. The downsides are low health and infrequent turns, but the pluses are ranged damage, which is bad for ranged units (and Academics already have 3 ranged units in their army!), so you have to shoot at it. Besides, it's also very dangerous in melee combat. Already more interesting...
Emerald Dragon: This is a mixed bag. On the one hand, 100% is weak, it dies very quickly (and not beautifully), but on the other hand, it can be used as a scout, thanks to its acid breath, plus high initiative * (specifically) for positive effects.
Black Dragon: Similarly. Only in reverse. Clearly too strong, also high initiative + acid breath, but now magic resistance can really hinder it. Give it "protection from negative effects," and that's it. But no, there's no way to increase the strength of this monster, which is bad. That's why I take red ones, but that's another story.
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Lava Dragon: I liked their resilience. I remember testing them against a black dragon, and the latter barely won. Very, very useful dragons, almost unkillable, with burning breath, and also with runes!
Cyclops: At the moment, I'm playing as Orcs, and that says it all. Thanks to their rage build-up, these guys forget what death is, they are quite strong, they like to break the necks of various Chaos Devils and the legs of Titans, and most importantly, they can also shoot. =) The only problem is that when they lose all their rage, they become slow and weak and die quickly. (I'm not even mentioning the uber-mace - no friends, no enemies, I'm standing alone in the field).
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Result: From the poll:
1) Chaos Devil/Infernal Devil
2) Titan
3) Archangel
4) Archdevil
5) Ghost Dragon
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Overall result:
1) Cyclops (- loss of rage)
2) Lava Dragon/Chaos Devil/Infernal Devil (- no speed/no runes) (-magic immunity) (-weak)
3) Titan (-slow)
4) Archangel (-no damage range, weak resurrection)
5) Archdevil (-weak, ugly, something is missing)
6) Ghost Dragon (-weak, takes a long time to build, no pluses except 3 per week)
Ps: I'm voting for Chaos Devils, just so they can be on par with Titans. =)