Personally, I think that the Grand Conclusion would be a very, very long and bloody battle to the death between the Orks, and the Tyranids. Why? When you kill an Ork, spores fly off, and several more Orks appear in it's place (After a bit, I suppose), along with a group of Gretchin and Snotlings. Wiping Orks off the face of any planet is, barring the Exterminatus, or the whole "We're eating your planet, sucker" thing that the Tyranids do. Assuming that it's a case of everyone versus everyone, the Orks outnumber... Everyone. Barring Tyranids, possibly. So, they (Orks) would eventually wipe out everyone else, and when the remaining Hive Fleets come out of the void? There's a worthy foe. The Tyranids get more guys when they kill someone (Take matter, back into the hive fleet, and turn it into More Tyranids) and the Orks get more when they die (As I previously said), so who stands a better chance?
In conclusion - Chuck Nor*Sound of sluggas/shootas*
In conclusion - Chuck Nor*Sound of sluggas/shootas*