I'm gonna have to agree with Star Wars as a victor. As others said, the technology and numbers involved is just too superior. I dont know anything about Warhammer so I could be wrong.

However, the Starcraft versus Halo fight is a much harder call. Let's break this down shall we?

STARSHIPS: Human

Anyone who actually looked at the cinematics in Starcraft would realize the battlecruisers are about the same size as a UNSC longsword. Both UNSC and Starcraft ships have one-hit one-kill weapons (MAC and Yamato Cannons respectively.) Both ships are unshielded. In a raw capital ship battle, it could go either way. However, the UNSC doesn't have any kind of electronic warfare ships that could compare with a Terran Science Vessel's capabilities. The EMP pulse and support shields of a Science Vessel alone would give Starcraft a good advantage.

STARSHIPS: Covenant vs Protoss

Looking at pictures and cinematics, I'd say the Covenant capital ships are at least 33% bigger than a Protoss Carrier. The Carrier doesn't have any substantial offensive capabilities outside it's automated drones. The Covenant have massive lasers that could easily tear through a Carrier's shields, in my own opinion. In a raw fight, the Covenant would win easily. However, the Covenant don't have anything like the Arbiter-Class Protoss ship. A cloaked Carrier Fleet supported by Arbiters would be tough. This fight could go either way.

STARFIGHTERS: Human

If I had to venture a guess, I'd say the firepower of a Wraith and a UNSC Fighter are about the same. Again, it comes down to cloaking capabilities that the Wraith has over the UNSC. A Pelican Gunship would not even compare to a Valkyrie Frigate. This is an easy win for Starcraft.

STARFIGHTERS: Covenant vs Protoss

We simply don't know much about Covenant fighters but lets just assume the Covenant is better. If the Protoss Scout is weaker than the Covenant fleet, but is supported by Corsairs, well anything could really happen. I'm calling it a draw.

GROUND BATTLE: Human Infantry

The Terran Marines from Starcraft are basically walking in heavily armored mech suits. Its been established that anything smaller than a 50 caliber gun would probably not penetrate that armor. The Terran Marine guns are literally 50 cal.'s unto themselves, and equipped with grenade launchers. Firebats have even heavier armor than the Marines and giant flamethrowers. Meanwhile the UNSC Marines are very lightly armored and carrying small arms. Helljumpers have only slightly better armor than the average UNSC marine. Yes, I know the UNSC Marines can get rocket launchers, but rocket launchers versus grenade launchers pretty much cancel each other out. This is a win for Starcraft... unless Master Chief is there. :) A much better battle would be Terran Marines versus Brutes.

GROUND BATTLE: Covenant vs Protoss Infantry.

The Protoss Zealot with its blades is basically equivalent to an Elite with an energy sword. A Protoss Dragoon is roughly equivalent to a Brute regular. A cloaked Dark Templar is nearly identical to a cloaked Elite. The Covenant have massive numbers of grunts, jackals, hunters, and flying bug things. The Protoss have High Templar which could probably take on all of those at the same time. I'm calling this as a draw until you throw Archons and Dark Archons into the fight... then the Protoss win hands down.

GROUND BATTLE: Human vehicles

 Seige Tank versus a Scorpion. Both vehicles have approximately the same armor and cannon damage, but the Seige Tank has it's secondary seige cannon. Add Vulture bikes and Goliath Walkers to the mix, and no sorte of scorpions or warthogs would save the UNSC. Starcraft wins again.

GROUND BATTLE: Covenant versus Protoss Vehicles

Well,  the Covenant tank has about the same firepower as a Protoss Reaver, except the Covenant have much much better manuverability. Covenant win, and if a Scarab enters the scene its a slaughter.

ZERG VERSUS THE FLOOD.

Zerg win hands down. Zerg are simply a much stronger and a more specialized race than the Flood could ever hope to be. The basic Flood form is about as strong as a Zergling, or maybe half of a Hydralisk, and we havent even scratched the surface of a Zerg Hive yet. Even if the Zerg allied with the Aliens from the movie Alien, it would still be a one sided fight. The Alien is only about the strength of a Hydralisk.

As you can see, with the exception of the Covenant ground vehicles it all either comes to a clear Starcraft victory or sometimes a draw. If the Starcraft races were united in a space battle it would be a slaughter of anything Halo could throw at them.