Neutrals, who are they?

Phoenixes – no comment.
Wolves are a very strong unit, IMHO. In the Campaigns, during the VS for Freya, either by script or by bug, wolves join your army. And I personally fought with them for quite a long time; they are not only tough, but also very strong. Moreover, the reinforcement stack is not considered summoned. If the "original" stack is completely destroyed, but the summoned one remains intact, the number of wolves will not change after the battle.

The most respectable elementals are the fire elementals. For their ranged attack and many shots...

Knights are also strong units. The developers skillfully place them in the Campaigns, for example, when they give you a hundred dragons in the VS. :)

Although, in general, all neutral creatures are noteworthy and are among the strongest creatures at their level (except perhaps not for all elementals).

PS Are there really weak versions of phoenixes and death knights in the game? I didn't know...

Phoenixes for their respawn ability and high initiative, death knights for their ability to destroy a large number of enemy units, mummies for their ability to resurrect. And I added yetis to the poll for no reason – they are not documented anywhere and were removed from the game (although they remained in the resources) in patch 2.1.