Skeletons win not only with their numbers but also because they have a large shield, magic resistance, and a stunning attack. There are far more of them than warriors, and they are easy to resurrect. In addition, they are immune to poison, mind control, and don't panic. There you have it.

Skeletons can only be resurrected with "Raise Dead," which reduces their HP each time. This will lead to a unit having only 1 HP, and they will be defeated more quickly.

They are not effective against gremlins, warriors, fairies, imps, or peasants. But in my opinion, the "best" unit of the first level is the Hunter because:
1. With invisibility, they can approach the enemy calmly (without a hero).
2. When attacking from invisibility, the enemy does not resist.
3. Each attack poisons.

At the end of the list, I would put goblins because of their cowardice.