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Judging by the poll, the monsters in Heroes 5 (6th rank) are quite evenly matched.
It seems that Ents are slightly behind, but as was noted here, there are 5 of them per week!!! The total amount of XP and all other defensive properties do not make them outsiders – I am sure that many are guided by an attacking strategy...
...of course, it is much more pleasant to quickly defeat a 7th-level monster with a sword than to stubbornly hold back the onslaught of a horde of monsters with Ents :)
P.S. I personally voted for "cave lords," but I respect all 6th-rank monsters.