I have generally found those black plate infantry with morningstars to be good castle stormers. The shield and protection protects from most arrow fire.
Warning: They (black plate infantry) are not very good at Ctis castles and caution should be used against strong nature nations whose defenders are mostly of the archer and mage variety. Breath of the Dragon, poison slings, and other poison attacks are devastating against Ulm.
The real problem here is that the combat AI will run you right through poison clouds you could easily avoid. It's painful to recall how many black plate infantry I once lost to a retreating hydra. There are some things you just should know better than to chase after, especially when you ought to know they're faster than you anyway.
I will usually send a group of vine ogres in first when I suspect the use of poison.
I'm sure black knights work well due to their mobility, but if you don't have them hold a couple of turns, they will draw the spell fire to themselves...and they are expensive units which hold up poorly against magic.
The real answer to almost all of these questions in Dominions is: it depends. (And I love that.
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What are the defenders? What is in the towers? (What is in the towers depends on the nation.) How many towers?