A few thoughts... (linked with fanatical defiance of logic and general hyperbole)
Or the other option for the Necrons... a single C'tan... consume the sun of the system, wait a few days, voila instant ice cube...
It takes thousands of years for a C'tan to consume a star. C'tan are natural beings, most stars have one, including Sol, most likely (though Sol's C'tan was never 'awakened' by the Necrontyr and so is completely peaceful.)
Consider the sun of the Necrontyr homeworld. The Nightbringer was constantly draining that for thousands of years and couldn't destroy it. He wasn't even close by the time the Necrontyr bound him in physical form, after which he decided to eat souls instead. Souls are like the junk food of the C'tan - they're not healthy or filling, but you can't resist eating them.
Necrons:
50 Cairn class Tomb Ships, 3 of them penetrated earths defenses, think what 50 would do. Also if they didn't just destroy the entire planet, lets see 1 normal army fits in that, so
Both C'tan 50 lords
100 monoliths
150 heavy destroyers
6000 warriors
3000 immortals
and just to kill psykers, 500 Pariahs ((Emperor (of corny names) got nothing on them), Feel my blasphemy!)
that should do it I think!
The three ships that penetrated Mars couldn't fight, they were at top speed the whole way. As soon as your fifty ships slow down, they've got all of Battlefleet Solar breathing down their necks. Need I point out that the three were destroyed as soon as they landed?
The C'tan - bah, what will they do? They're not gods, and their necrodermis shell isn't that tough to destroy (judging from the game, of course). A Titan shoots it, the necrodermis is disintegrated, and the C'tan, in its incorporeal form, floats back to a Tomb World in shame. They can't be actually killed, but they're near helpless to actually do some damage.
The rest - assuming a basic Necron Warrior is of equivalent skill to a Space Marine (given the sheer power of Space Marines, that's unlikely, but just suppose), do you really think that under ten thousand will acheive something a million Marines, supported by millions of Traitor Guard, hordes of mutants, and several whole Titan Legions, couldn't do?
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).
Terra is a full-blown Hive World. The average Hive World, according to the BBB, has a population of between 100 to 500 billion. Terra itself will likely have more than that, given its status.
Even if billions, even a few hundred billion, are scribes working for the Administratum, that still makes an incredibly large army. Heck, the scribes themselves get pushed into service as conscripts! Terra has the numbers on its side. Can the invaders match that? A Tyranid Hive Fleet, assuming they can land uncontested, might rival it, and a large enough Ork Waaagh! has the potential to get there, but I think it's safe to generalise that you're not going to outnumber the defenders.
Tyranids, apparently having some serious numbers at their disposal, would appear to have perhaps the best chance.
Maybe, if they can get the numbers there. As I said, you can't really play the numbers card - Terra can raise a larger army, and its elite troops are better than anything the Tyranids can bring to the battle. If you invoke the 'infinite Tyranids' argument, they're all right, but in a practical sense, I don't think they can do it.
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.
They couldn't do it without some major tricks up their sleeve. The defences of the Imperial Palace could happily chew up a Craftworld or two, so direct assault is out of the question. I don't think they could infiltrate their way in either, given all the psykers. It's very tricky - if you asked a Farseer if it was hypothetically possible, he'd tell you 'no way', but in a roundabout 'everything is possible' Eldar kind of way.
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.
Agreed. They don't really stand a chance. With enough Orks, it could be done, but that many simply couldn't be raised. If the Imperium crumbled, eventually attrition would mean that the Orks would have to win, but I don't think they could carry the day in a direct assault.
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).
So far they're the only ones to have tried it, and they did pretty well. They could have done it in the Heresy - if the Emperor hadn't killed Horus, and those Space Wolf/Dark Angel reinforcements weren't coming up, the Palace would have fallen.
Do they have the resources to try again? Not yet, definitely not yet. Terra is as strong, if not stronger, than it ever was during the Heresy, and they've lost too much. In the distant future, possibly in a second Heresy, maybe, but not now.
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.
Numbers. Not one Tau world has a population even approaching that of Terra, though the entire Empire probably exceeds Terra's. The Tau won't be ready for thousands of years yet.
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.
They can't do it. Their technology is advanced, but the Necrons can't fight wars of attrition - as far as we know, they have never fought to the end before. They can't handle the slogging match that a siege of the Palace would turn into.
Overall, I would say that Chaos has the best chance. In the present, however, no one can realistically hope to successfully storm the Imperial Palace. It's just too massive and too well-defended. It would take at least a million Space Marines, plus supporting troops in the form of billions of Guardsmen, and hundreds, if not thousands, of Titan equivalents. Can't be done.