If we ignore the Borg. Dominion hands down no question. The only reason the Romulans, Feds and Kilingons were not wiped off the map in weeks was because the wormhole was blocked one way or another. If the dominion was able to send reinforcements it would have been game over. And that was stated throughout the show. A dark horse that I think would be equal or better than the feds are the Kremin in the delta quadrant. Infact they are so close to borg space it look like they are pretty powerfull to have survived especialy with there advanced weaponry.