Cardassians--because they're awesome anyway, and because they got smart and rebelled when nobody ever thought they could do it. Well, do it they did!
At one hell of a cost. They may have the distinction of being the first and only race to shake off their Dominion masters, albeit with a lot of help.
I might have voted Cardie - but I don't see them as "real" members of the Dominion. Allies of convenience only, and temporary ones.
"Transporter accident." "Maybe you should go talk to Worf again."