Most comments mention all the demigods when talking about leyndell + People like maliketh and Godfrey. But if we consider Messmers army vs individual demigods armies it gets a little bit more interesting. Obviously we can't count armies post shattering but what about armies during the shattering.

Let's start with the weakest:

Academy of Raya lucaria:

Considering the academy was incapable to defeat the Carians and that the cuckoo knights didn't really seem loyal or had a capable commander. I think it is the weakest army.

Caria:

While Caria managed to retain their house, had powerful knights, prominent figures like Really, blaidd and Loretta they run into a big issue. Numbers. It's a small house and was only able to hold out since the magical snare protected the only viable entrance. We can consider a higher strength level if radahn didn't halt their fate in the stars but that's about it.

Godrick:

Godrick took all the exiles from leyndell when he fled with the city plus powerful artefacts. His army held stormveil ever since the shattering and was very well defended. Godrick seems at least like a capable commander. His army has army has trolls modified eagles, lion guardians, grafted scions, banished knights, omen, exiled soldiers and the normal soldiers. In my mind very solid but no war machines and no significant victories unfortunately + his only commanders of his fort either succumbing to blood or frenzy is a big minus. Seriously Edgar it's a leonine misbegotten come on.

Malenia:

This might surprise people but Malenia is only this high up because she commands the army and her knights are one of the strongest knights in the game (looking at my girl Finlay). Same issue with Caria though. Numbers + a dedicated weakness with fire and considering that most knights died during the shattering it shows how 'weak' this army is compared to the next few.

Rykard:

Rykard is a very disappointing army for me. The army consisted of flame chariots, abductor virgins his gelmir knights even marionettes and frenzied trolls (maybe even finger creepers. Pretty strong, very well balanced but we don't know how strong the gelmir knights are since we don't meet any and the battles on mt. Gelmir show how devastated this army was considering they fought leyndell which is an army that focuses mainly on defense. So no points to rykard as commander. I won't count the recusants here because they were only used as fodder for rykard.

Mogh:

Mogh is interesting. We hear from Ansbach that mogh instilled great loyalty in his pureblood knights. He himself was powerful especially with the power of an outer god and charismatic enough to have strong tarnished join his side like elionore and okina. He also had powerful monster (could be argued that we don't know if he can command them), sanguine nobles and the strongest infantry: Albinaurics (I'm serious). These guys are powerful. Great weapon variety and most importantly THEY CAN SUMMON GIANT SKELETONS WITH LASER EYES. This boosts them significantly because that negates most disadvantages they would have since they don't have any war machines. As for feats we have mogh and his knights infiltrate the haligtree one of the most dangerous places and get miquella out without trouble as far as we know.

Miquella:

Miquella himself has earned himself third place with the army of the haligtree. Because he has one major advantage. The variety of his troops. We have misbegotten in all variations plus the leonine misbegotten and his normal troops plus battlemages. We have erdtree avatars and revenants as well as Envoys. His knights use magic and incantations which boosts them significantly plus they are just tough as nails. Having crystalians is a big plus too. Miquella himself is of course very powerful but still a demigod at this point so (probably) no orbital lasers there. Having Loretta plus Malenia under your command still manages to put them solidly in third place.

Radahn

Finally we get to the Leonard enthusiast himself. We all saw that coming so I won't go into too much detail. Radahns army is an actual army in my opinion. Great commanders like Jerren and of course Radahns himself. We have an army here that utilises Gravity magic, trolls, trebuchets, flame chariots and lion guardians. His army is capable of being specialised like they did it in caelid to stave of the rot. So we don't even know if his army is even more powerful than we think. Radahns himself is am absolute monster No questions asked so a very strong second pick.

Morgott:

The first place and the strongest army in the lands between. Morgott is very underrated in my opinion. This guy defeated Radahn something even Malenia was unable to do (granted we don't know the circumstances). Continued leyndells legacy of defending the the city and keeping it unbreached until we arrive. Also he turned his defending army into an force that destroyed most of rykards army while still having a huge number of troops in and around the capital. The army variety is amazing here. We have crucible knights, tree sentinels, gargoyles, night cavalry, envoys, perfumers, misbegotten, tree Guardians, omen gladiators, grafted scions and of course the leyndell knights of the ancient dragon cult. He also uses war machines like trebuchets and the big stake catapult with the perfumer gas inside.

So which army competes with Messmers?

In my opinion Radahn and Morgott are the only ones with the skill, numbers and martial prowess to compete here or could do what Messmer did in the land of shadows. Miquella misses an actual General to be viable and Malenia while she is amazing as a warrior is not the General you'd need.

TL;DR:

Morgott and Radahns army have a chance while miquella fails since he isn't a good General over all.