Well, uh... There are a few things to pick from.
Ermor is definitely swampiest of the swampies, but their "decent" units are kinda hit and miss. And, as mentioned, needs a bless.
Nazca (not sure if that's what you mean by "mummies", since C'tis also has mummies and one could make an argument for Ashdod's undead summons) has a whole bunch of chaff options, from slaves to reanimators to just a bunch of cheap easily-massed basic troops. For their decent dudes, they've got those ethereal flying sacreds you can summon via a spell or just spawn for free, but like Ermorian lictors they're mainly only noteworthy if you've got a nice hot bless for them to wreck things with.
Without a bless though, those mummy bundles are absolutely terrifying battlemages. Not particularly mobile, but they have "innate spellcaster", 0 encumbrance and generally good paths. There's even a hero bundle who is capable of casting 3 spells a turn like it was no big deal.
Then there's stuff like Shinuyama's cheap bakemono-sho grunts, and their excellent sorcerors, plus other battle-ready commanaders and some decent elite troops. There's also Ur, which has recruit-everywhere everythings so you can amass a horde across multiple provinces, although as far as "decent units" go it's kind of a gray area.
EA and MA Pangaea can send freespawn maenad swarms, and Pan always has some good troops available for heavy hitting... LA R'lyeh gets some freespawn swarms too, but not to as huge an extent as Ermor. That said, R'lyeh's decent troops are... Seriously terrifying.
Xibalba has massive swarms of recruitable troops, but not a lot of elite units outside of summons.
I guess it kinda depends on whether you specifically want a freespawn nation like Ermor, Lemuria, Asphodel; reanimator/summon nations such as Lanka, C'tis, Nazca, Mictlan; or just readily-recruitable horde nations such as Ur, Xibalba, Shinuyama, Jotunheim or Bandar Log.
Heck, there's even stuff like LA Ulm with wolfherds/vampire counts making their chaff, backed up by the always exemplary Ulmish footsoldiers and knights.
...and then there's Therodos. Nobody knows what the hell Therodos is.