What commandment from God is the toughest to comply with?

I am an atheist, but your 9th or 10th are the hardest. It is often difficult to not lie in circumstances in which there would be repercussions for me telling the truth. As for the tenth, it it human nature to be greedy, we always want something we don't have. It is incredibly difficult to be content with what you have, because you will always see something else that will pique your desire.