This thread is very silly. It's full of people going "a bajillion of everything my army has!". Let's face it, Terra is not going, in itself, to be phenominally guarded. It's surrounded by a large number of heavily defended systems, so by the time a hypothetical army gets near Terra, it will have had to fight through a heck of a lot of other stuff. Terra isn't the biggest planet out there, and has limited capacity, so it will not rely on having massive numbers of defenders, but rather a fair few incredibly potent defenders (Adeptus Custodes spring to mind).
Tyranids, apparently having some serious numbers at their disposal, would appear to have perhaps the best chance.
Eldar, I think we agree, would be unlikely to manage it at this stage.
Similar applies to the Dark Eldar, and this isn't the kind of thing they'd be interested in in any case.
Orks, should a Warboss be motivated to get enough Orks together they'd probably have a go, but they aren't really sophisticated enough to try anything beyond "get em boyz", which is probably the first thing the Imperium will have thought of to defend against.
Chaos seem to be another likely threat. Of course, the numbers of marines have depleted a great deal since the Heresy, so they'd need to try something pretty sophisticated (which they are certainly capable of).
Tau, another contender, they're expanding a lot and getting gradually stronger. They also have the benefit of sophistication and tactical nuance to find the best possible way to do it. I favour these guys almost as much as Chaos and Nids.
Necrons, short of pulling the Outsider and the Dragon out alongside a lot more Warriors than have ever been seen before don't seem to have much chance.
All in all, I'd put my money on Tau, Chaos and Tyranids if anyone were to manage it, with Nids coming out on top.