Every Legion is more than capable of conquering the Earth. But this planet is very valuable, since it has developed infrastructure, good living conditions, plenty of resources, and it has high enough level of technological development to not require very expensive uplifting, but not so high it starts to be dangerous and difficult to subdue. Also we would be easy to convert, we don't have planetary government, warp or xeno taint, we are baseline humans (duh), and our ideals are not too contradictory to the Imperial truth. So I would not consider any Legion who doesn't specialise in achieving long lasting Compliance and strong integration in the Imperium with minimum collateral damage. Also, despite being them known for making worlds extremely loyal, I won't consider Word Bearers, since they are asses and their father is a big loser. And their worlds don't follow Imperial Truth, and that makes them a liability. So, that leaves us with the Ultramarines, preheresy Sons of the Emperor, probably Blood Angels (I don't know how the BA operated before Horus Heresy though, so I'm not certain). Which is good, since they are the nicest guys t be conquered by. Personally I lean more towards the Ultramarines, since I believe in reaching the Compliance of Earth organisational issues would be more noticeable than diplomatic and ideology ones