The most difficult alternative

Death, in my opinion. It's difficult to choose at all levels, especially at the third. Both the offspring and the vampires are very strong! With Nature and Might, the choice is difficult at the 4th level.

in a game against a human player, the choice is clear, because movement speed on the map is always important, in my opinion:

with Death - 3rd level - vampires are definitely the best, and the 4th level - depending on the circumstances - both skeletons and devils are good.

with Nature - phoenixes are definitely the best - against a human player, they are almost always superior, and they are also faster and more resilient;)

with Might - you can think about it - if you need defense, choose hippogryphs, and for long journeys, to minimize time spent, choose thunderbirds.

It's difficult to single out any particular race. But the main dilemma, in my opinion, is monks and crusaders. And I'm not the only one who thinks so, apparently.

Well, this is hardly a dilemma - crusaders are definitely the best, and only if the 3rd level unit isn't going to be used in the final army (which is often the case) can you choose monks - they cost less, and it's easier to clear the map with them. You can also take them if you have an order mage with accuracy - but again, it's only a less troublesome option for clearing neutral creatures. And if you can't get accuracy, then monks are useless, in my opinion.