Replicators. They'd actually EAT the Borg, wheras the Borg would be forced to deal with them the way the Asgard did. The Asgard lost, and they wouldn't have been considered food the way the Borg were. Generally speaking, the Borg would lose to almost all of their sci fi counterparts - they were tough in their universe, but they share the weaknesses of both a biological and machine race for having the strengths of both. While they're invulnerable to most energy weapons and can survive vacuum, they remain quite vulnerable to old fashioned projectile weapons, biological warfare and abominations such as the Replicators. Their ships were durable, but lacked the sheer firepower that ships of similar size had in proportion.
SPARTAN291
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