I'll cast a vote for the Breen energy disrputing weapon.
Sure, it won the Dominion a battle or two, and delayed the Feds and Romulans for a while, but presumably the Dominion asked the Breen for assistance on the assumption that their technology would turn the tide, or at least delay the Alpha Quadrant powers long enough to allow them to retool, rearm, and launch a new counter-offensive. Instead, they once again underestimated the resourcefulness of their opponents, while simultaneously helping to alienate their Cardassian allies.
Not the best of strategic moves, if you ask me...and considering the losses incurred at Chintoka, the weapons performance afterwards was subpar at best.