This talk of the Necrons penetrating Mars is BS. Your making it seem like they secretly penetrated the world and oh noes! The imperuim was so helpless zomg!!!1!
As I remember it, you had a Necron ship moving really fast. So fast it's weapons couldn't even fire, and all it could do was fly in. The MINUTE it stopped, it was blown out of the sky. Oh yes. That scares me so much. They can fly around mars all day, but the minute they stop they get blown up?
And even if they took mars, so what? It's 1, count it again, 1 forgeworld. The Imperuim has hundreds of thousands. Would it be a blow? Yes. Would it spell the deathknell for the Technocracy? No. Losing a capital is really not a deathstrike. It effects morale more then anything seriously concrete.
As to "Necrons could invade terra from mars!" arguement, I don't know. Horus failed to take Terra and that was with a huge amount of support from many many planets, AND the imperuim was locked in civil war so Terra could recieve very little aid. He only managed a stalemate, and when the imperuim did recieve aid he was forced to...well...we all know what happened there ;)
The Necrons, being equipped with only A planet full of resources, would find it very difficult to invade terra from mars.
My Vote for most dangerous goes to Orks.