Rampart - I don't play.
Tower - Solmyr (chain lightning is very helpful here!).
Inferno - I don't play.
Necropolis - Tant.
Dungeon - Zheddit, Malekid.
Stronghold - Krag Hack, Shiva is also good.
Fortress - Broolhyd (wyverns are the foundation of this castle; I build them in the first week. Also, giving heroes with defense skills levels is always a good thing, and it's nice to have minimal losses after a battle).
Conflux - I don't play.