Xenomorphs, Necromorphs, and Headcrabs do not stand a chance in hell against any other singular unit in that competition, are you serious? Headcrabs especially because they lack any special traits whatsoever and are poor combatants on their own merit. Headcrabs would be the first to go, if not then it was because they were deemed such a nonissue that everything else ignored them.
Xenomorphs and Necromorphs are also nonissues for the other four races/organisms presented. Xenomorphs, while deadly on their own merit, lack the numbers of Tyranids, Zerg, and Flood, as well as lack the adaptibility of the The Thing. Besides, Genestealers from the 'Nids are just as deadly, probably even more so. So Xenomorphs are loving out. Necromorphs are just reanimated corpses so that's just a bunch of slow, shambling biomass for everything else to consume. So those are out.
That leaves Flood, Tyranids, Zerg, and The Thing.
Now the most comparable of those four would be Tyranids and Zerg, and I'm not afraid to say that the Tyranids do literally everything the Zerg is capable of but better, faster, more efficiently, and with greater number. So Zerg are out. This leaves Tyranids, Flood, and The Thing. Due to the infectious nature of the Flood it could be argued that Tyranids and Flood would be on somewhat even ground here. However, assuming the Tyranids would be capable of ingesting Flood tissue without infection, the Flood are gonna get forgeted.
So this, therefore, leaves the Tyranids and The Thing. Judging by what Zombi is saying, The Thing is the most tangible threat against the Tyranids, however I don't believe he considered that if The Thing were to disguise itself as a Tyranid, it would be found and left away from the main force, just like Primal Tyranids, due to its lack of connection to the Hive Ship. And assuming it began to attack the Tyranids the Tyranids would fight right back. But considering what Zombi is saying I don't think the Tyranids would be able to kill it dead, and it would continue to be a bit of a thorn in the Tyranid's side for a very long time.
tl:dr Tyranids win, The Thing is still alive. Everything else gets to get shat on. That's my input anyways.
Also only like four of these are parasites what are you doing OP
They are all parasites.
Other than that, LACK OF ADAPTABILITY?!
The Thing can stay frozen for thousands of years and still live.
Even when it is killed it's never fully "dead".
Personal Opinion on the War:
Headcrabs and the Xenomorphs would be caught up a little, but regardless, Headcrabs would die off first.The Xenomorphs would get infected by the Martian Gothic Bacteria, instantly making groups of three Xenos fuse together into a Trimorph.
The Trimorph and Non-Dead Xenos would fight the Necros, carrying the infection. Necromorphs get infected aswell.
Necro-Xeno-Undead Trimorphs move onwards.
The Thing, although having very little hosts, assimilates the bacteria, eventually getting its traits, and then assimilating the Xenonecromorphs.
The Thing assimilates them and moves on to The Flood.
The Flood and The Thing have an endless genetic war, constantly regenerating their genes.
The Thing would probably allow itself to be assimilated, only to assimilate The Flood from the inside out. (Probably)
The Xenonecromorphs assimilated by The Thingflood, moves on to The Tyranids, which have already assimilated The Zerg.
Thingflood assimilates Tyrazerg over hundreds of years time.
Thing/Flood wins.
(In my opinion)
