In a fight between the UNSC and the Last City, the UNSC would most likely win due to sheer numbers, a [u]much[/u] larger army, and a large powerful navy. Although average guardians are considerably better fighters than average UNSC marines and ODSTs, and would have far higher kill counts, they would be far too outnumbered to make a true difference. Plus, spartans and guardians are very equal fighting forces against each other, in my eyes. I'm gonna pit the Covenant [pre-2552] against the Cabal. In this face-off, I think the Covenant would [u]easily[/u] win, largely due to similar reasons however. Although we haven't got an accurate estimate on how large the Cabal Empire and Military is, I have a strong feeling that the Covenant is much larger in almost all respects. Their navy is stupidly huge, consisting of millions of vessels (their largest ships measure almost 29km in length. I mean, c'mon...), and their army consists of billions of assets. Honestly, billions. I can imagine the Cabal to also have a large Empire, but it would pale in comparison to the Covenant in my opinion. Oh, and also, the Covenant glass entire planets with ease. Next, I'd pit the Flood against the Hive. Again, I think the halo universe easily wins this one. I understand that the Hive are a deadly force to be reckoned with, having access to their own throne worlds and arcane sciences. But, even then, I think the Flood are still far more powerful. As far as we know, the Hive only defeat enemies though fairly conventional and typical means (invading planets with armies and guns), but the flood are far more sinister. Their ability to assimilate living bio-matter would be a death blow to the Hive. Not to mention that they gain the knowledge of any assimilated hosts (thus meaning they'd learn everything about the Hive), and the gravemind can even become psychically powerful enough to access ancient networks like the domain. Put that into practicality, and I can imagine the gravemind accessing and invading entire throne worlds. As for the Forerunners, I'd have them go up against the Vex. Now this face-off is one that I'm quite undecided about. Both races are obviously mysterious and highly advanced, and I think both would be fairly matched. However, we just don't know enough about the Vex for me to make a final call. It's unlikely that the Vex could be harmed by the halo array, given their mechanical nature (and yes I know they have biological elements but an organism needs a developed nervous system to be killed by the halos; something the Vex don't have). In the end I'd have to make this face-off a tie due to complete lack of information on the Vex, concerning potential strength and abilities. You'll notice I haven't mentioned the Fallen or the Reef Awoken. This is because I think that these two factions are far too small and weak to be worth talking about in any great detail. I don't think that either the Fallen or the Awoken could beet any faction from the halo universe. In the end, I think the general verdict is that the halo universe would win. BUT this is largely due to a lack of information concerning the Destiny universe and the capabilities of the Destiny races. Truthfully the only way I can see the Destiny universe winning is if some idiot activated the halo array and wiped out both sides. Both the Exos and the Vex would likely survive the array's firing (and I can imagine the Hive surviving by retreating to another dimension/throne world). However, the Forerunners do have shield worlds to protect inhabitants from the array, so much is still up to debate. Holy crap it's actually quite hard to choose a winner. And I didn't even mention the precursors and their abilities.