The Dominion. In tech terms they are equal to the Federation, but in terms of numbers, military capacity and arms/shipbuilding, few can rival them. All they need is a localised Founder to command operations, as well as his or her Vorta to command the Jem'Hadar and any captured/occupied worlds, and then one is set.
The Federation was lucky to win the Dominion War anyhow. Had Sisko not been the Emissary, and had not tricked the Romulans, I'm positive the war would have been lost.