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Guys, what's the point of comparing units one-on-one when they have completely different growth rates and survivability in the game? Plus, we shouldn't forget about the unique characteristics of heroes from the respective factions. Within these parameters, I'll share my own opinion.
Level 1: As a rule, only skeletons and imps (which are carefully protected) pose a serious threat, so they are the strongest. Even a powerful assassin, due to his meager growth rate and distance penalty, is sometimes useless, despite the poison effect. And if the opponent is undead, then it's game over...
Level 2: The Fury, beloved by many, undoubtedly dominates, although in a prolonged game against the Arabs, there might be too many of them.
Level 3: In defense and tactics (especially when killing superior neutrals), ghosts are the most useful, but in a pure PvP setting, hunters are unmatched.
Level 4: With small numbers, druids are effective; with large numbers, upgraded lizards (primarily) and griffins, sometimes vampires. Succubi die too easily and suffer from simple weakening, which is the main bane of Inferno.
Level 5: Unicorns and archliches clearly stand out, and depending on the hero, hydras can be effective. Inquisitors and djinns are not very impressive.
Level 6: Everything is very diverse and balanced, although as the number of units increases, cave lords, like all casters, perform much worse, and the slicing sword ceases to play a significant role.
Level 7: All are good except for the spectral dragon, which is not saved even by its increased growth rate. And the most balanced unit seems to be the Black Dragon.