So, here's the deal:

Peasants are only good for sitting in Elain's treasury and bringing in money, and after they get a halberd, they become even more insolent and stop bringing in money, instead opting for a weak stunning attack. So, if you're going to upgrade them, go straight to archers. In my opinion, they're useless on the battlefield.

Imp, Fiend: a suitable combat unit for the first level, but they're useless in battles with neutrals... And it's hard to keep them until you encounter an enemy hero. Therefore, you have to buy them in limited quantities and use them sparingly. But against another hero, they'll show their true worth – if the hero is a knight, barbarian, or elf, then the enemy's spellcasters are doomed.

Scout, Assassin, Hunter: here, here, here – say hello to the best unit of the first level, no arguments needed. They've already been praised enough, I'll just add that Scouts don't have a penalty in melee combat – this is very useful for clearing out neutrals at the beginning!

Fairy, Dryad, Nymph: of course, they're good – they fly far, have high initiative, can quickly deal with neutrals with their ash swarm attack at the beginning, and have spell removal (I think it's cheating to give such spells to a level 1 unit), and nymphs heal thanks to druids. But... they don't live more than 2 turns – they're fragile... and even the fact that they aren't attacked doesn't save them. Fly to block archers – they'll be taken down in one turn. But in my opinion, they're second in my ranking.

Skeletons: The days when hordes of archers were the mainstay of the necromancer's army are gone, but not everyone is quick to dismiss them, and rightly so. Because you still recruit a lot of them, and they shoot quite well. As they say, at first, it's not a strength, but soon there will be VERY many of them anyway, and the new pink skeletons are even better. I can't provide evidence here – I don't play as necromancers much, but in the first mission of the necromancer campaign in the Horde of Orcs, I had a hard time with neutrals and skeletons.

Goblins... Probably the most ambiguous unit of the first level. They shoot weakly (in my opinion, with the same peas), and their stats aren't great, but their abilities are helpful, especially the new one, they can quickly cripple the start of machinists and even damage upgraded machinery for a turn. And the old golems (especially when upgraded) repair golems, which can be unpleasant.

Again, I won't say anything about barbarians, although I prefer them with traps. At least you won't be hit by a spell. In my opinion, the first level of barbarians is not very successful.

Mountain Guards, Warriors: third in my ranking... What they do... oh my god... And with Ingmar, they're terrible units. Shield wall is an excellent ability. It's hard to break through. Again, sorry for the lack of evidence. Everything said above is based on the feeling of playing AGAINST dwarves.