By race:
Devil - Skeleton:
If you compare hit points and damage, the devil looks weak. But if it comes to speed... it's especially interesting when fighting non-ranged and not-too-fast neutral creatures. Demons take care of things while the devil himself sits in the corners of the battlefield using teleport. At the same time, he can also deal damage (don't forget about freezing!).
Fairy - Phoenix:
It's complicated. Very complicated. Resurrection or mirror image? Spells or speed? Besides, many resources are needed for the forest, but not in very large quantities, while a lot of mercury is needed for the stronghold, which is much harder to accumulate. But then the mantis comes into play... the portal itself is relatively cheap, and there are many other creatures in it, and there clearly won't be enough money for the mantis and someone else from this pair (But the mantis can bind, and considering that you need to stop the troops on their way to the druid... By the way, if I have to choose, I prefer phoenixes.
Golem - Titan:
As much as I love dragons, I have to say goodbye to them here. Even considering the first strike and the fact that it's a mechanical unit... Someone said that there will be too many ranged units? Golems and illusions can hold back enemies.
Angel - Champion:
I don't have any ideas here, because when playing as Life, I usually don't get this far. But when I do, I'll prefer angels according to my favorite principle: "Better to have a few crocodiles than many mice."
The same goes for the dragon-hydra pair. Although the hydra attacks everyone and without retaliation, the dragon has immunity, breath, and many other abilities. And it looks great!
What did I forget... hippogryphs-thunderbirds: birds are clearly faster and don't slow down the hero on the map. And they will deal with ranged units like those of the Order. The behemoths are, of course, more powerful, but they barely crawl to the battlefield. And just their appearance...
Well, who is the strongest... mega-dragons and fish are not available, unfortunately. But even super-duper black dragons had to share the spot with insects. Because of the cost and the ability to bind