The perfect Necropolis.

In an ideal Necropolis, there should be one unit for each (or at least many) unit of other factions, so as not to turn griffons into vampires and do similar things. And, of course, ideally, human skeletons should differ from, say, elf skeletons in greater resilience, but less so compared to dwarves, although this is already a very ideal scenario, and it is possible that these would be unnecessary complications, too complex for practical implementation.