Out of the choices the Galaxy Class, after the Dominion upgrades is the strongest. The Steamrunner Class, after the Dominion upgrades, would tie the Defiant for 2nd in my opinion. The Steamrunner/heavy frigate class is designed to take damage and, with ablative armor, could hold it's own against the Defiant long enough to damage back, though the Defiant is a good mix of attack and defense. The Galaxy Class wins out of the three simply because it could put out and take more than both.
The Sovereign Class would trump all of them though. The Defiant would be blown to pieces against Shinzon's ship.