Screw the Mursaat, everyone thinks they're so cool but Margonites are the best. They're fluffy, snuggly, adorable, and... oh, never mind.
I think the Margonites+Torment Creatures+Lords of Anguish+Mallyx+Abaddon would win. As Zahr said, Margonites can bring in Harbringers, which means that if you don't have vials of purified water you're kinda screwed. Abaddon's Chosen (or favor?) makes you immune to losing health for 10 seconds... goodbye Spectral Agony. Margonite dervishes can really rip you up (I just noticed how powerful they were taking my monk through Nightfall, two of them can pretty much destroy the backline) especially if they're in large groups - they have party-wide healing, good AoE damage...
There's also the Torment Creatures, which are like the Margonites' "pets" in a sense. Spread out your damage the wrong way and the mob can double... then quadruple... and increase exponentially in size. Rain of Terrors are absolute killers - Maelstrom to shut down casters, Blurred Vision to shut down fighters, and Mind Freeze + Deep Freeze to tick you off.
The Lords of Anguish... The Dreadspawn Maw probably could keep spitting out Stygian creatures and Torment mobs, The Fury can... uhh... I'll get back to you. I have no idea what the Greater Darkness does, but imagine Lord Jadoth riding into battle on Mallyx the Unyielding. I'd love to see Lord Jadoth riding Mallyx... if you take my meaning
If Mallyx doesn't want to be mounted (if you take my meaning

) he can still kick ass on his own, I mean he's a great big 35 foot tall Margonite gorilla. He can eat enchantments and hexes, shut down shouts, chomp up conditions, and fling you around his Citadel like a banana. Plus, he hits 400+ damage with his normal attacks...
And if any of the other armies dared to have a dance party, Abaddon would pop up beside them, do
his little dance, and they'd get served.
Margonites ftw <3