As we all know, the Imperium as may enemies, both within and without. But who poses the greatest threat to bringing the Imperium to it's very knees?

Personally, my money is on either the Tyranids, since Leviathan is still going strong towards Terra and some believe that these Hive Fleets are merely the vanguard to a much bigger force.

Or the Imperium itself. It's human nature to destroy ourselves and I can see the Imperium getting crushed under it's own weight.

Agree? Disagree?
Your thoughts please.