40ks biggest assets will be Dark Age Humanity, Pre Fall Eldar, War in Heaven Necrons, Old Ones, and Chaos. Each of these individually could defeat modern 40k, and they each pack some serious weapons. The biggest threat to them will be the Precursors, but we have little information on them, and their offensive capabilities, so I'll just stick to Forerunners and their ilk. Necrons at their height could match Forerunners at their height around 1:1 when it comes to planets, and they also have the backing of the C'Tan, which are effortless reality warpers. The C'Tan would make mincemeat of any Forerunners they encounter, because they are basically real space gods. The Necrons also have antimatter guns, shards of neutron stars they launch at nearly super luminal speeds, dimensional phasing, matter to energy conversion and more. Dark Age Humanity had continent sized warships with guns that could create black holes, advance entropy, real space FTL, and more. Pre-Fall Eldar likewise had a huge empire, and were capable of moving stars around like toys. They can also predict the future, trap black holes in a box, revive the dead to fight in their war machines, and their psychic powers basically bypass all Forerunner tech (Nice armor you have there, too bad you brain just exploded, and your blood boiled.). They also have the Webway which allows them to rapidly relocate, hide, and attack from. Old Ones are a wild card, but they created and uplifted many species to use as weapons of war and had complete mastery of the Warp. Chaos is a huge threat to Halo. Halo has absolutely no experience with something like Chaos, and this would leave them wide open to corruption. Not to mention the war would fuel the Gods to even greater heights. The Flood is a massive tribute to Nurgle. Halo also has no weapons to effectively combat Chaos. They have no knowledge if the Warp, and the idea of a place where time can be a solid, nonexistent, or flowing backwards would be difficult to comprehend. They are a science based race, now they find themselves up against magic. As for the Flood, 40k has synthetic units to attack them. A massive army of Necrons, Men of Iron, Wraithguard, Daemons, and C'Tan would give the Flood a hard time, because they have no biomass to infect. It would be a war if attrition. And with the Daemons, C'Tan, Men of Iron, and the new Wraithguard (thanks to the Forerunners fighting flesh and blood Eldar), it would be an uphill battle. If worst comes to worst, the Necrons could use the Celestial Orrey and enact a Scorched Earth policy, aided by the Eldar's seering and their own predictions. Time travel would help plug any leaks in the 40k lines.

Lul warhammer Love halo more but it would be a noob to say so? 100,000 space marines vs master cheif , lol 1 space marine could quite likely screw him up let alone 100,000 Nids will devour the galaxy Neurons will kill you on there own?! Serious warhammer destroys halo in a fight?

People keep saying Precursors, the Forerunners beat the Precursors so they aren't that tough. Also 40K has Necrons, Tyranids, Orks, Chaos, Space Marines, trillions of imperial guard and not to mention their gods... so 40k beats almost anything. Loling at the fanboys

40k wins this has been known by everyone that isn't stupid for a long time.

ITP - halo fanboys It's well known that 40K would obliterate almost any other universe.

[quote]and is at their prime[/quote] Even if it was just straight up 40K vs Halo, it would be a massacre. If you are giving 40K all of it's races in their prime... lol. They'd need a new word because Shrekoning doesn't quite cut it anymore. Eventually it would just come down to the Gods of 40K vs Precursors which is an unwinnable stalemate. The only one that I've seen so far as actually beating 40K in terms of ridiculous concepts/OP as phuq weapons is the Whoniverse.

Every species other than the precursors and WH40K's God's would Die. Then it would be a draw. What Mr. Psychologist said. Edit: I forgot to mention that precursors and anything they have can only be defeated by neural physics, so does WH40K happen to have any neural weapons at their disposal?

Precursors solo.

The point of 40k is to be way too overpowered so that it is dominant universe in video games. 40k wins

Master Chief solos