<p>Unalaq with or without Vaatu? If with, then he would own that fight, hands-down.
</p><p>If without, I would say Amon.
</p><p>Kuvira has demonstrated some pretty damn impressive metalbending skills, but she's no match for Amon. When we first saw Kuvira fight, the first thing she did was immobilize her targets with metal restraints. Yakone, a bloodbender on par with Amon, was able to incapacitate an entire room full of people, including Toph (a metalbender even more skilled than Kuvira), all while he was handcuffed. Even Aang needed the Avatar State to defeat him. Amon was trained by Yakone and is arguably an even better bloodbender, compounded by the fact that he can nullify his opponent's bending abilities. Kuvira might last a bit longer if she had her super-mecha-spirit-cannon death machine, but it would do her little good against more mobile adversaries. I'm guessing that Unalaq would find a way to neutralize any dangerous spiritual energy that the weapon possesses, and Amon would relieve her of her bending soon afterwards.
</p><p>Zaheer is a very skilled airbender, and is even deadly enough without his airbending abilities to warrant being locked in a supermax prison. Add to that the fact that he can fly and project himself into the Spirit World, and he is a worthy opponent for anyone. But his only real offensive weapon aside from a few forceful gusts of air is the ability to literally suck their air out of a person's lungs. He would undoubtedly be an elusive target, and he might even come close to suffocating his opponents, but he would have to get close to them in order to do it, and that's not going to work out well for him.
</p><p>Unalaq is really the only opponent who can truly be a match for Amon. He is a waterbender, and thus the only one with any hope of a real defense against bloodbending. I would also argue that Unalaq himself may be a bloodbender...perhaps not in the same league with Amon, but still enough to give him a shot at winning the fight. As a former member of the Red Lotus, he has associated with people who were skilled at unorthodox bending styles (lavabending, combustion, etc), so it's reasonable to assume that he has at least a basic grasp of bloodbending, or at least enough to limit Amon's hold on him, much the same way that Amon was able to defend himself against Tarrlok's bloodbending. Aside from that, his conventional waterbending abilities were impressive enough to hold his own against the Avatar and his brother, Tonraq, another exceptional waterbender. But unless Unalaq has an army of spirits he can call upon (which is doubtful if Vaatu is out of the picture), then I see Amon as the only one left standing at the end of this fight.
</p>