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Definitely not Necropolis, many alternative units are better (skeletons, vampires, dracoliches), and liches are better in their older versions.
Definitely not Inferno, again, many alternative units are better (demons, succubi, hellhounds), possibly also dogs and pitfiends, but it depends on the situation.
Definitely not Stronghold, also several units are not very balanced (centaurs, shamans, wyverns, cyclops).
Regarding the remaining factions.
Humans are relatively balanced, perhaps archers are not very good, and monks are not always useful.
The Sylvan faction has almost all units that are good, but the archers' upgrade is very unbalanced.
In the Dungeon faction, lizards and maybe witches are questionable, otherwise, they are acceptable.
In the Academy, only rakshasas are not very balanced.
The Fortress faction doesn't have a major imbalance, but berserkers and dragons might be slightly overpowered.
In conclusion, Humans or Academy, probably, or maybe Fortress. It's hard to say for sure. Necropolis and Inferno are the most unbalanced, and Stronghold is not far behind.
Definitely not Inferno, again, many alternative units are better (demons, succubi, hellhounds), possibly also dogs and pitfiends, but it depends on the situation.
Definitely not Stronghold, also several units are not very balanced (centaurs, shamans, wyverns, cyclops).
Regarding the remaining factions.
Humans are relatively balanced, perhaps archers are not very good, and monks are not always useful.
The Sylvan faction has almost all units that are good, but the archers' upgrade is very unbalanced.
In the Dungeon faction, lizards and maybe witches are questionable, otherwise, they are acceptable.
In the Academy, only rakshasas are not very balanced.
The Fortress faction doesn't have a major imbalance, but berserkers and dragons might be slightly overpowered.
In conclusion, Humans or Academy, probably, or maybe Fortress. It's hard to say for sure. Necropolis and Inferno are the most unbalanced, and Stronghold is not far behind.