I'm pretty sure I have heard homilies in Mass on all of the commandments at one time or another. I think the hardest to obey would be to not bear false witness. We gossip so easily these days, sometimes before we even realize it. I would say that none of them are least difficult to follow/understand. It takes daily work to follow God and to remember his commandments. It takes practice to follow them.
The commandments have set a precedence in our man made laws we follow. It is illegal to kill and steal. Growing up, lots of stores were closed on Sunday (though we already discussed that was not the intended Sabbath day) because people were going to church. You can get in trouble for saying false things about people.