Tapeworm
Tapeworms are a type of flatworm that are parasitic. It is precisely the fact that they are parasitic that makes their nervous system so interesting. It's not that they have a better one than the other flatworms, but that they have a reduced nervous system. Its main cluster of nerves is in its scolex, or head. Besides that they only have a small network of nerves throughout their body. This is because if you are a parasite, living in a safe, warm, and food rich environment, there is really no reason for you to have a complex nervous system. Tapeworms have such a reduced nervous system, that they don't even move. Without the need to react to threats and or to do any complex thinking, the tapeworm nervous system has just been reduced over the generations.