All castles in Heroes 3.

I don't know who is stronger... it's objectively very difficult to say.
I'll say that I've always liked Nighon.
Minotaurs (the best), Black Dragons (maybe Archangels are better in some ways, but the immunity puts Black Dragons ahead)
Portal of Summoning (often useful in campaigns), Mana Vortex... and it's aesthetically pleasing.