The BORG by far, they had their transwarp hub, they could be anywhere in the Galaxy within seconds. The only defense the Dominion could have would be The Founders/Shapeshifters infiltrating a Cube and doing damage that way. This is no small treat, however, in the long run. I believe that the BORG would overcome the Dominion. Just look what happened at WOLF-359.
At times they have almost been defeated by species 8472 with Voyagers help, however, the BORG are a very resilient foe, espically with the Queen seeing and hearing all within the collective. And having the ability to respond in seconds to any threat anywhere within the collective is a pretty compelling argument for the total BORG victory.
The Founders better just stay put in the Omarion Nebula with the Vorta and Jem-Hadar and hope the the BORG never pay them a visit...
Resistance is Futile