I'm going to have to go with the Borg here. Even if they can't assimilate, they've still got massive numbers and firepower on their side. One Cube is enough to engage a good chunk of Starfleet for three days, and that's just a standard Cube, not an Armored Cube. Factor in how many vessels the Borg are bound to have in their fleet, and you've suddenly got a force that's both unstoppable and immovable.
However, it is important to note that the Federation's trusty defense against the Borg --remodulating shield harmonics-- were useless against the Dominion. I'd love to see how that plays out.