Carthage.
As pointed out, as dangerous as the Germanic tribes were, they ultimately made the Empire fall only after the Empire itself suffered economically and with endless civil wars. Persia, also as nefarious to our Greek-speaking heroes in the eastern Empire as it was, was never really able to push into the Empire except once.
Carthage, though? Carthage was an existential threat. They could have actually destroyed Rome as a nation and concept and at the heigh of its patriotism and military fitness - razing it, eliminating all its territory and loyal allies, freeing resentful subjects. It's telling that out of all Roman enemies, Hannibal was literally used as a boogeyman by Roman mommas to scare their behavior into being better.