2 Ley

Well, I wouldn't say that... If the developers set prices reasonably, then they definitely took into account the cost of building one or another dwelling.
Why, for example, does a Cyclops King cost (as far as I remember) 1100 gold, while a Wyvern Monarch costs 800? Even though the Wyvern is stronger.

Well, for starters, the Cyclops has ranged attack, and the Wyvern doesn't :)
Then, the developers were more likely considering the total cost of buying units for a week. All Castles (except the Tower) completely buy out all the "available" units, having only 1 Capitol.
Some prices may have even been artificially inflated a bit to prevent giving an advantage to the Human faction.

2 Ann

Dark Knights, of course... Nagas are also good, and Cyclops...

There's nothing particularly great about Cyclops, IMHO :) If they're supposed to break down walls, then what are flying and ranged units for? And wasting precious turns on ranged attacks on walls doesn't seem like a good idea to me :) Besides, without Ballistics (and Ballistics, IMHO, is one of the weakest skills), while they're breaking down the walls, it's easier to "clear" the Castle with them...
And regarding attack, I once calculated..., and it turned out that these same Cyclops have a lower average damage output than Grand Elves.
I calculated it like this: (Damage * "attack" difference * weekly growth).
------------------
In general, I would give third place to the Human Cavalry.
Average stats, like the Knights, but if you play skillfully, you can get a 25% attack bonus almost every turn (5% bonus per tile).
Fourth - Unicorns. A 20% chance to blind isn't decisive, and you have to blind ranged units specifically to make them "shut up".
----------------