Regarding Death Knights, I think it's appropriate to use them against Sylvan (you can place Death Knights next to the cavalry, because you can use the Death Knight's ability on a unit attacking the cavalry, and the Death Knight can attack before the cavalry and clear the path for the cavalry! IMHO).
Dark Knights against Mages, Necropolis, Inferno, and Humans! IMHO.
P.S. I haven't decided which dragons to take against Stronghold and Fortress, please advise? Also, tell me which witches you use to fight neutral creatures?