The most useful monster.

The Dragon King is a very good unit! Its area of effect attack can harm your own troops, but it can also be very helpful – for example, when storming a castle (the Dragon King attacks not only the gates but also the allies standing behind them). And it can also be blessed, unlike the black dragon (although that one is also incredibly overpowered with its stats). But... there are situations where the Dragon King may not be useful despite its mega-stats. An example? Sure. I'm creating a campaign and naturally testing it as I go. I had a situation: a hero, 3 Magic Dragons, and 5 Dragon Kings against another hero (level 1, without an army – just marking that the hero will be there). The Dragon Kings didn't make it in one turn. Then it was the Magic Dragons' turn, and they lazily blasted with lightning. Only bones remained of the hero. I prefer mage units, but ones that don't fall quickly in melee combat. I also take archers to thin out the enemy army before it reaches my troops. Although... a strong infantry unit wouldn't hurt. The usefulness of a unit depends on the player's tactics and how suitable the unit is against the opponent's units (for example, a vampire is very useful against the basic Nature army, but it clearly has nothing to do against golems, undead, or elementals). But in any case, a pumped-up mage hero or a hero with Warfare (and tactics, plus troops) will definitely not be useless in battle.

but for me, the most fun is an army of only heroes and a large supply of potions of immortality...

Yeah, that's pretty cool, I have to admit... But even one well-leveled hero with an immortality potion is more than enough – I've seen it firsthand