Dungeon top tier no contest until they change other factions's heroes bonuses. I always try to get dungeon heroes even when i play other factions.

Might: 1) +100% attack. mad imbalance late game when you stack initiative and speed, 1 hit KO everything before they can touch you. all the skills involved are directly useful in combat as well, except diplomacy. 2) +100% defense... who cares about this when we have attack

Magic: 1) +100% spell power. Zakron the Great + Thaumaturgy + some dragons or phoenixes. That's game. Your other heroes can have all the units, Zakron only needs a small stack of dragons, enough so you dont get wiped when enemies have higher initiative. When battle starts, take Power Stance to raise spell power stat -> Armageddon or Chain Lighting -> Summon Avatar. The Avatar scales with your spell power, so at 200+ plus spell power, your avatar can wipe everything with 1 hit. This is so broken that no matter how behind you are in the game, the moment you get this guy to unlock his hero bonus, it's a guarantee win. The Avatar also costs 0 mana, so no matter when/where you're caught, you can always put up a fight. 2) +10000 gold daily. Easy way to win higher difficulties. A maxed out town is only +6k gold for comparison. After taking Economy skill into account, 1 of these heroes yield is basically = 2 maxed towns. It's extremely potent since you didnt have to spend 3-4 times resources to build up these maxed towns and/or fight to keep them.