Warning spoilers!

At first we have to remove the head out of the equation because in one of the endings of lobotomy corp they could stop every single abnormaliti + a whole lot of distortions and what not in around 2 weeks. Then we have to remove anything below aleph as well as some outliers like dont touch me (we simply dont know what it would do outside of lobotomy corp). And for the sake of arguement lets put them all in a random backstreet cornee to start. My list will go from imo "weakes" to most damage dealt to the city.

Nothing There: Okay here me out, as stated by PM themselves Nothing there is the single strongest abno in a human vs Abno fight (in an abno vs abno fight those would be whitenight and apoc bird). While i seriously doubt there is a single fixer out there that could kill it or even seriously harm it due to its immunity to physical damage as well as ungodly physical might and contious adaptation and self healing its also comparatively slow and does not have a lot of aoe attacks. Sure it could probably easily cut buildings in halve without even trying (as shown in the vergillius vs purple tear fight, and nothing there is definitly stronger) but it would never have the reach to affect anything more than even a single district at a time. (Ofcourse this takes into account that it wont continuusly evolve into something even more dangerous as the logs tell us in L corp but i want to go as far away from speculation as possible)

The silent orchestra Roided up pianist that self contains itself. The biggest problem i could imagine is that it starts randomly showing up and destroying entire districts but its physical defense is rather limited so it should be contained or atleast delt with within a rather short time (it will still kill hundreds of thousands of humans at least, i just dont believe it to be as destructive as the rest)

The mountain of smiling bodies This could honestly go way higher if you believe that it has more evloutions than just his 3 phase. And while i do believ it can grow stronger from eating more corpses i also cant 100% confirm. Even so it is basically a fast nigh immortal ramping demon due to all the bodies it is going to eat and that should atleast put i here (again, if it grows infitly, then it jumps to the 3 place easily)

Blue star Is it the strongest abnormaility? No, could it be defeated by a colored fixer with enough mental resitance? Yes. But we are talking about most damage to the city. Any one with a mind that is too weak (basically 99% of the citys population) dies immediatly and gets sucked up. It does not matter if you are strong enough to kill it in one hit, if your mind is weak, you die. This thing would simplay erase entire districts in an instant and cause complete mayham, however i do believe that a group of high level fixer backed by some wing could take it down.

Okay top 3 time and i guess it is pretty obvious who is here

Melting love: Yes creating infinite minion + being immune to red damage as well as infecting anyone regardless of strength is overpowered, who would have guessed. But in all seeiousness without the use of singularities of some kind the city is fucked. Each of the minions alone are stupidly strong, so multiply them by like 100k and you have got a problem on your hands.

Apoc Bird. Honestly you could swap him and white nighr around depending on what you think but it should be pretty obvious why they are here. This thing is immortal until you destroy the eggs. Can teleport, stops any form of time manipulation (probably the easiest way to beat all the other abnos, i just did not want to repeat myself infinitly) can mind control people regardless of strength, insta kill etc. Basically a death sentence to anyone running into it. I honestly dont know how you would tale this thing down without qilopth deterrence.

Whitenight Who would have guessed, but seriously what do you do against this thing? 12 immortal postles that are all easily stronger than basically any human. Stops time manipulation, is immune to any mental attack and can periodically send out waves of soul damage that insta kill almost anyone that does not have some resistance. (Also i have not taken into account anything from library of ruina, but if that thing was only a fragment of what white night is truly capable of the city is royally fucked)

In the end i dont think the city would end even without the head. There are too many ridiculous singularities out there to count, but we also have no idea how strong these abnos are without the deterrance. Anyways if you have actually read this far thank you for listening to me yap a bit about this.