Every one of these organs are very important and they each play a role in the human body. You need your brain to think and it's with your nervous system and you know what that is. Your heart pumps blood to all of your veins and arteries which flow throughout the body. Your lungs makes it possible to breathe. Your stomach holds your food you consume which then goes to the small intestine and large intestine. Without any one of these, your body can break down.
I say brain, without the brain, you can not process information. Lets say you move you arm, your brain sends signals to it telling it to move. Without the brain you can do basically nothing. The heart is also very important, but is not as important as the brain. If the heart has a failure, there is artificial hearts can be used, not as good as a real heart of course, but without the brain, what would be the purpose of the heart? Yes, it is to pump blood, but without the brain you aren't really there.
Hard question. Your heart would never beat without the brain telling it to, and the heart pumps blood in order for the brain to make this command. Without the lungs,
you wouldn't have the oxygen for either of these things to happen, and the stomach provides the food that converts into energy to fuel these events. Impossible question to answer.