I've gotta go with the Dominion.
The Dominion are close enough to Borg space that the two have likely been well aware of one another for quite a while, if not centuries. If the Borg had felt they were capable of subduing the Dominion, they would have likely already attempted it.
I'm sure any such war would be *incredibly* long (it would make the Alpha Quadrant war look like a fleeting thought)
, but the Dominion are incredibly ingenuitive and resourceful. And I would bet that, in its home quadrant, the Dominion war machine would be next to impossible to keep up with.
I wouldn't go so far as to say that the Dominion could defeat the Borg. But I'd say that I don't think the Borg could defeat the Dominion.