Interesting question. And a tough one 🙂 There are lots of different nerve cells in the body that all do really important jobs. One called a DRG is responsible for letting you feel everything from touch, to heat, to whether you are full up after dinner. The ones in your spinal cord are responsible for taking all of those senses back to the brain and for delivering messages to your body and the nerve cells in the brain called neurons are where all your thoughts and senses actually happen. If I had to choose from these three I would say the neurons in the brain, because without them you would be dead, but I will actually choose a different one, one that we don’t really have any more. It is a type of stem cell called a neural progenitor cell and it makes all the different types of nerves that we have. We don’t have many of these after we are born but they are easily the most important because they make all the others. Did you see on the news about the paralysed dogs walking again? That was done using a type of nerve stem cell so they could be really useful in the future.
I don’t think there is one most important nerve cell. The most important thing about nerve cells is that they connect together to make us able to do all the amazing things we can. One nerve cell on its own would be pretty useless!