Auto-translated
They underestimate the Mage's ability too much. Together with the Wizard's Mark, it allows you to cast two Berserks, two Resurrections, two Divine Intervention, and summon a full-fledged Phoenix. You can cast Holy Word or throw a Fireball/Meteor Shower, finishing off a vulnerable stack. While not 100% efficient, the Mage can react to any event during the battle. For example, if an enemy runs towards your archers, the Chaos Mage can quickly try to throw stones at him, while the Light Mage can cast buffs and just pray. For Darkness, you might face Necromancers, and without Sandro's Cloak, you'll have problems.
What about resurrecting creatures, when it seems that it's enough for the hero to finish off the stack and have the next unit in the turn order move into its place (especially if the enemy hero with resurrection is in front).
And all this is possible when you still have 2 skill schools available. Yes, you can't get a Lucky Wizard there, but otherwise, you are almost not limited.
In addition, you already have Enchantment and Education. Machines may not be very useful, but the ability to capture an enemy machine is quite good (not saying it's great, though). Together with Gremlins, you can immediately disable the machine build, as well as the tent and the ballista.
Plus, as I said before, there are two schools. You can develop Fire Warriors, or mass Light/Darkness. Or just Attack + Defense, Morale for the mini-units, and you'll get a combat build capable of mass magic. Only Demons on Gating can have a similar level of freedom and variability – when you can direct the battle the way you need.
The other abilities can be countered (when you reach them), but Mages and Demons, in theory, cannot. There is only one option with Demons, where Orcs have a talent for damage to summoned creatures, but besides that – there is practically nothing. Even if you counter them with slow and other effects, you will still have 100%+ of the army of gated Demons in front of you. And the damage of various banishments is laughable.
The Barbarian is also not bad in terms of versatility, but you need to accumulate troops for a long time, and when the army size decreases, the strength of the ability starts to suffer significantly.
So, I would rank them as follows:
1. Mage (two available schools, which can be used to achieve the desired result, plus maximum variability in the entire game)
2. Demon Lord (but only in the case of a fully specialized gating build, in other cases – on par with Necromancers)
3. Barbarian (If the Demon build is not for gating, then the Barbarian becomes second)
4. Runepriest
5. Ranger (due to lower versatility, as it is easier for a Gnome to compensate for attack than for an Elf to compensate for defense, and there are fewer build options)
6. Warlock (Chaos can finally participate in serious battles, but is still vulnerable due to its uniformity)
7. Necromancer (Morale is cool, of course, but not entirely versatile – there are builds where the effect will not be so noticeable for the enemy, even for magical heroes)
8. Knight (His ability is more of a balance, as the Knight himself is an ideal commander, and can, in principle, become a pleasant addition to an existing build, but I'm not sure if that's his goal)