I voted Hirogen...in complete disregard for the rules of this poll.
You see, the word 'toughest' to me doesn't imply 'who would win a war', it suggests who would possess the physical and mental toughness to prevail.
Too many of the races mentioned are only formidable due to their tricks. The Founders are shape shifters [I see no real evidence of 'toughness' from them], the Doud...we know very little about, what we do know is they are reclusive explorers. [not 'tough'] The Borg are bad-ass due to their technology and fact that most of them are mindless drones.
The Prophets are god-like and frequently stand-offish.
So I vote the Hirogen. Who are frequently up for a fight, will relish it and have the physical and metal stature to continue it.