What is the most important lymph node?
I do not think that a most important lymph node can be named. There are hundreds of lymph nodes spread throughout the body. Their primary function is to filter the lymph that is being circulated throughout the body. In addition to the lymph and the lymph nodes, the lymphatic system also includes the tonsils, adenoids, spleen, and thymus. I honestly don't think that I can pick a single most important organ among even those four. The reason is that they all work together to keep the body running at peak performance. The loss of any of those organs is detrimental to human health. It's possible to remove any of those organs and still live, but the immune system is weakened.
If I were forced to pick which of those four lymph organs is most important, I would pick the spleen. It happens to be the largest of the four, but that is not why I would pick it. It controls the amount of red blood cells and blood storage in the body, and it helps to fight infection. Blood is extremely important to the body. It carries oxygen, cellular nutrients, hormones, and waste products, and helps heal wounds, so controlling the amount of blood and blood storage is very important to the human body.