This one kills you. Highly, highly poisonous. From Wikipedia:
Initial symptoms after ingestion include severe abdominal pain, vomiting, and diarrhea which may last for six to nine hours. Beyond these symptoms, toxins severely affect the liver which results in gastrointestinal bleeding, a coma, kidney failure, or even death, usually within seven days of consumption.
The Registry of Toxic Effects of Chemical Substances assigns psilocybin a relatively high therapeutic index of 641 (higher values correspond to a better safety profile); for comparison, the therapeutic indices of aspirin and nicotine are 199 and 21, respectively.
Essentially impossible to overdose on. The only adverse effect from this (or any psilocybin) is the possibility of a bad trip. But normal usage is virtually harmless.
Guess which one is illegal.