The Q continuum. They are able to change history as much as they want, although they don't do it most times. They could provoke war against any life-form at anytime, anywhere.
From the "normal species" (such of no god-like attitutes): Species 8472. The only species that had won almost against the Borg. The Borg hadn't survived, if Voyager hadn't helped them.
Were you trying to say "attributes'?
The Q Continuum are the most powerful species. If you're not counting the Q as they're "gods", you could say the Prophets. The prophets don't have much power outside of the wormhole apparently though, and are considered gods. The Borg, although the Federation would stand absolutely no chance against them if they launched a full invasion, are still not the most power "ungodlike" species. Species 8472 could of destroyed the Borg, but the USS Voyager helped them since they saw Species 8472 as more of a threat (the Borg didn't exactly decide to send an entire fleet of cubes to the Alpha Quadrant did they? At least there's that. Species 8472 would just send a fleet of bioships and probably destroy all the homeworlds of Federation species). If you're only counting Alpha and Beta quadrant species (Dominion was from the Gamma quadrant and not a single race, Borg from Delta quadrant and Species 8472 from Fluidic Space), probably the Klingons. Think about it, the Federation is a collection of worlds, the Klingons are only one race and yet they still stood a chance against the larger Federation.
the bajoran prophets seem to pocess similar powers to the Q's but how they compare to the Continuum is unknown
If you are asking what the most powerful state/empire is then I would have to say the Borg.