Probably either the Dominion or the Klingons
It depends on what you mean be "strongest". In terms of physical strength or martial prowess I would agree that it is a tie between Klingons and Jem'Hadar. In terms of weaponry the cardassian disruptors are thought by many to be the best in the quadrant and Cardassians have been describe a "worthy enemies" by Klingons a statement they do not make often. As far as technological superiority the Borg are the clear victor.