The most difficult choice is probably for Necropolis, because all their units are equally valuable. The Ghost is, of course, weaker than the Cerberus, but it has excellent speed and ages enemies, and its necromantic abilities will only increase. Then there are the vampires and poisonous creatures, which are also roughly equal, and the poisonous ones are real monsters when defending a castle, although I always choose vampires because they are easier to use to clear the map, unless you encounter a lot of golems and skeletons along the way. And the 4th-level units are also diverse – on the one hand, Devils are good for quickly eliminating a hero, especially an unarmored mage, but on the other hand, Bone Dragons are very strong in melee combat, and if you also capture a nature castle... :D
In conclusion, all the units are good, but I always end up building only necromantic units: Ghost, Vampire, Bone Dragon.