Provided none of them have allies and Unalaq is not the Dark Avatar
Amon>Unalaq/Kuvira>!Zaheer
Amon's bloodbending is ridiculously overpowered and he can also psychic bend, although the show came up with a bullshitty way he got defeated (how exactly did Korra escape his grip again?) he is hands down the strongest waterbender and his bloodbending abilities don't even need him mobile - but he is pretty hard to pin down in any case, showing impressive dodging skills many times.
Unalaq is a waterbending master and I would say whether he beats Kuvira depends on the environment. If he has plenty of water to draw on, I personally think he could defeat Kuvira as he seemed to do pretty well against the Avatar team with only a little waterskinful of water in the prison, admittedly Korra knocked him over pretty decisively in the end but it seemed to me him running out of water was the reason behind this. Later in the season he still kicked Korra's ass with only waterbending. Kuvira beat Korra whilst she was still weakened, sure, the poison was out but even without that her mental state would have still greatly limited her abilities even outside the Avatar state; when Korra regained her power she was defeated in a fair fight.
Zaheer can fly, sure, but he only beat Korra when she was minutes or less away from death anyways. Tenzin is a stronger airbender than Zaheer (minus flying, but having one specific skill doesn't always mean you are stronger in general; Zaheer achieved flying due to his mental state, not skill) and all of the other three have ways of countering him should he fly in to attack; Amon can just bloodbend the shit out of him, Kuvira has shown her ability to immobilise people with metal and could shoot those metalbending wire ropes at him. Again, Unalaq might be fucked if he didn't choose his terrain (hence the !) but in a place with a lot of water I don't think it would be a problem for him to just encase Zaheer in ice.
Maybe I am underestimating Zaheer and the power of flying (perhaps his attacks could have enough range to attack from where the others can't hit back? although again, other benders have shown ways of getting high up in the air, too, if only for a short time) but he never seemed like a very strong bender to me (I mean, he is certainly very strong compared to the average bender, but compared to the other "masters" he is not very strong imho), which doesn't make him a bad character or villain ofc :P
Well in a situation where Unalaq WAS fused with Vaatu I'll assume his "Dark Avatar State" would negate Amon's bloodbending like the normal. (Though not confirmed.) But just because he ca negate Amon's bloodbending wouldn't mean Amon would automatically lose, yes Unalaq is a master Waterbender but Amon is a "decent" Waterbender and has amazing hand-to-hand skills. He could most likely dodge/counter Unalaq's Waterbending easily. Kuvira and Zaheer would lose against Amon if he used bloodbending. Kuvira and Zaheer most likely could take Unalaq though. And in a Kuvira v. Zaheer situation it depends on how Zaheer defends against Kuvira's Metal Plates and how well Kuvira can trap an Airbender.
1. Amon (unbeatable)
2 . Zaheer
3 . Kuvira
4 . Unalaq Normal
Zaheer and Red Lotus are overrated.. Their debut was hyped but in the end it was the most embarrasing ending for any villain. Ghazan's end was dragged, Ming Hua's funny, P'li 's gross, Zaheer's embarrasing..
Kuvira is underrated here. Kuvira held her own against two Bei Fong (finest metal benders on earths and only earthbender that above'em is Ghazan imo) with relative ease. Kuvira able to hold herself against Korra after learn metal bend. Zaheer would be dead against metal bender Korra so many times. He even beated by almost dead and broken Korra rather easily.
Zaheer humiliated by Tenzin but same Tenzin was afraid of Amon.
So imo;
Amon > Kuvira > Zaheer after flight > Unalaq >= Zaheer pre-flight
FanCanyon, I kind of disagree with you saying that Ghazan is the best earthbender. It's clearly toph, but that's beside the point. I also don't think kuvira could've beaten zaheer because if she could, then why didn't she do it? Plus, kuvira lost to avatar-state korra almost instantly and was literally about to be killed by korra until korra's hallucinations stopped her which kuvira can thank zaheer for. Zaheer was already one of the most dangerous criminals in the world without bending and now that he's as light as the air he bends he can cross large distances in seconds, probably moving fast enough to dodge the metal projectiles kuvira uses until she runs out of them. I also think that no one on this list would've stood a chance against amon except if you're taking unalaq to also include unuvaatu and if unuvaatu doesn't have blood. So here's my take on this: Amon > Zaheer > Kuvira > Unalaq
Well, actually, it might be that unalaq's soul-destroying technique can have a huge area of effect depending on how much water he's using in it, in which case it's different.
Dude, while I haven't had time to post here recently until now, I've been noticing that you have a habit of double- to quadruple-posting. Why not simply edit your first posts instead, like most message boards/comments sections/etc. on the internet allow? In case you didn't know, the option drops down when you click the "More" button.
Anyway, while a Vaatu-fused Unalaq is almost easily the most powerful villain in the entire franchise (especially in his "UnaVaatu" form), I think Zaheer is the most interesting villain in TLoK.
I forgot that "who is the best" might not mean who would win in a 1v1v1v1. To me Amon seems like the most interesting villain because remember when he said to the benders of republic city "it seems fitting that you celebrate three bullies who cheated their way to victory..." but I also think kuvira's discussion with suyin was interesting because she basically said: (We're a sovereign nation. You can't interfere in our internal politics as if YOU had the right to appoint OUR leaders). Unalaq and Zaheer don't seem that interesting to me.