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Let's start analyzing this...
Dragon and Jelu are certainly good, but IMHO, some simple heroes, when heavily upgraded, are much stronger than any of the suggested ones...
I don't remember... I think Thazar has the "Armorer" ability? Something like 5% damage reduction... Now, think about it: will Jelu's snipers or Dragon's Wizards be able to save such a heavily upgraded hero with a sizable army?
Or, consider simple heroes who have the "Master of [Monster Type]" ability, where they get an additional bonus to defense and attack for each level (not all of them have this)... A direct example of this is in the later missions of the campaigns, where, for example, for Dragon in the final mission against a single Lazias, you can place one Titan, and he will defeat it with some difficulty, but he will still manage...
Out of the options suggested, I'm voting for Crag Hack because of the 5% attack bonus...
P.S. I voted for Mutare a long time ago...
These are my opinions... if I'm wrong, please correct me; I might be mistaken because I haven't played Heroes for over six months...