For me, it's the Borg. What they do to you is worse than death. And they can turn you back on your former friends and family.
That's true, but the Dominion was get points for determination, they seemed to want to conquer the Federation more then the Borg did. The Borg only sent one ship to assimilate Earth. The Dominion threw a ton of resources towards the objective of conquering the Federation. The Dominion engaged in total war against the Federation, the same cannot be said for the Borg.