All undomesticated animals would be the worst if they were not spirit animals.
Actually think about it. Every animal that is undomesticated will be wild and untamed and willing to kill a human whichIwouldn'tblamethemforcausewetreatthemliketrash. Unless they were absolutely non-threatening and boring, such as a blobfish. (Also, by the way, they would look like normal fish and probably do fine in combat if battles were fought 4,000 feet below the ocean. They just look weird and useless on land).
Any animal has the capacity to kill you, really. Even tiny animals, if there are enough of them. Or, they could be like a mosquito and carry yellow fever and malaria.
Worst for convenience would most likely be large marine organisms, such as whales and sharks. Certain types of marine mammals can survive on land, but others, such as whales, need water around them even though they also require air to breathe. Sharks would just wither and die in your room and be eaten.
Anyway, I agree with Happy. The worst spirit animal to take home would be a boyfriend or girlfriend.