Asphodel, Therodos (in theory, anyway
), LA C'tis, probably Nazca (though iirc their reanimators are kinda' expensive) would be the major ones, near as I can tell. LA Argatha might be a less numerous contender.
Anything that has access to reanimating priests or an equivalent, really, or those few that have national undead summoning spells they can actually spam. Conceptually, anyone with death access (to at least D2 or so)
can mass undead, it's just significantly more expensive for 'em gem-wise and notably less sustainable than those that have freespawn and/or reanimating priests.
As for sceleria, haven't played it terribly much, but from what I understand it's mostly just "Keep recruiting priests,* keep having them bring up longdead, bury everyone under a wave of corpses that eventually starts outproducing Ashen Ermor." Sprinkle with skelliespam assassins, gigantic communions, and something along the lines of great frothing hoards of astral artillery (1/4th of your grand thaums can mind hunt or VotD), and you've got most of sceleria's sceleria. Honestly, they're probably the game's least fiddly/most straightforward undead massing nation, it's just wrangling all those priests is something of a nuisance.
*Cultists if you're cash strapped/don't have a lab in the province, lesser thaumaturgs otherwise, indie priests
everywhere you don't have forts and they can be recruited. Every H1 priest is another 3 longdead (more if you give them the reanimation amulet whatsit) per turn for as long as they live! Get lots. You probably want gold scales just to keep up with the priest recruitment.