Borg no question, though for long time villains I much prefer the Dominion.
Twice single cubes have attacked Federation space, twice only stopped due to not the technology of the Federation in any way, but the being Locutus and his connection to the collective (in First Contact I thought Picard could just detect where the ship was actually really damaged at, not a hidden weak spot , like the Death Star), so it means first a battle would have to develop enough to have a weak spot develop, and then have someone who is connected to the collective hear where that is, and still have enough forces around to damage that enough to destroy the ship.
On VOyager we see the Borg get defeated by an extra dimensional threat, we see a cube get destroyed thinks to the actions of a hybrid drone from 29th century Federation tech and Borg technology. Voyager was capable of taking out a cube that was so damaged that only 4 or 5 early stage drones were left (out of tens of thousands), We say Voyager use first hand Borg knowledge, and Fed science to avoid destruction, but it was the collapse of the transwarp corridor that destroyed the Borg following them, not the weapons of the Federation. I honestly can't remember Unimatrix One, parts I or two so don't know how Voyager survived, and finally Voyager using future technology, was able to destroy a Hub, and the various other ships.
Which was really the only time I can remember them actually doing real damage to the Borg. Besides a small scout ship. That scout Ship was the only functional Borg vessel modern Federation technology was able to destroy on its own, I believe.
Just because Voyager survived, long after it should have been destroyed, didn't mean m Voyager was able to actually seriously damage the Borg, they were just able to flee fast enough away from them (in some cases by Borg science, by Kes' massive abilities, ect.