The Fallen are the only faction which has presence in three destinations: The Earth, the Moon and Venus.
They are able to fight the Hive:
Being one of the most dangerous enemies, having: Thrall, Acolytes, Knights and Wizards on their lines ready to strike down anything that is on their domains.
The Vex:
The most advanced faction of all, and not being able to defeat simple organic enemies that don't have strategies?
Having on their side: Hydras,Minotaurs, Harpies, Hobgoblins and Goblins it doesn't seem fit.
The Reef:
There's not much information about them, but they hate the Fallen and are able to strike them too and take precious targets, but the Fallen are able to strike back.
And the most important: The City and the Guardians.
Apart of fighting the Hive and the Vex, the Fallen dared to strike the last safe city on Earth, and treating the survival of the humans.
Which means that the Fallen have a huge army, being able to withstand all of the multiple attacks they have on all the fronts and survive being ready to attack once again.
The Fallen are more threatening to the Vex than the Cabals which supposedly are highly militarized and have a huge fleet too.
Why I say this?
Every time I see Cabal fighting Vex, there are only Goblins and Hobgoblins fighting them, sometimes a Harpy.
But the Vex take Minotaurs and Hydras to fight the Fallen, they even strike the Fallen on their own bases to take down the Anchor Priest, which means that he was a serious threat against the Vex, not even the Cabal threaten them that much and the Cabal were at the entrance of the Black Garden.
Apart, the Fallen are the only ones which I have seen to have a huge fleet having also huge ships capable of interplanetary jumping.
Probably the Fallen is our greatest threat.
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The fallen are split into multiple factions or houses though. Hey are not a unified military power but smaller factions. Also if I am not mistaken I think that there is often infighting involved between the different houses.