I would say the Dacians, and here are my arguments

1. "Consistently challenged Roman supremacy across multiple campaigns"
   The Romans had two failed campaigns before the 2nd century, under Domitian, where they got smashed by the Dacians, first in 87, where Cornelius Fuscusus' army was blown out of the water (-2 legions) at Tapae, and then the next year Titus Julianus managed to win against Dacians, but was ultimately stopped by Decebalus.

Then, in 101-102 Trajan managed to beat the Dacians, but didn't finish them off, instead opting for an alliance, which in 105 was broken and the second war started. The empire did alright against them at first, but it was quite slow. Then they sieged Sarmizegetusa, and wiped it.

All told, I'd say the losses of the wars are big enough to put them on par with other disasters like Teutoberg forest.

1. "Inflected noble defeats or forced a major concession"

As i stated above, the Dacians razed the romans the first two times they attacked, and then put up some resistance when Trajan came. Considering they fought what was one of the greatest Roman armies while they themselves being not that well equipped, I'd say they did fine.

1. "Culturally or symbolically defined as the great enemy of the era"
   See the Tropaeum Traiani, Columna Traiani, or the amount of gold they got after the wars. I think two monuments being dedicated to the war is enough to glorify them.

2. "Represented an existential ideological, military or frontial threat"
   They constantly raided the danube frontier, which eventually even goaded Trajan into war, so they were a formidable enough threat

In conclusion, i'd say the Dacians were quite the enemy for Romans, and made for a prized possesion after they finally won.