Depends on the situation.
If you are camping outside in a Tent, Bears are nothing to mess with.
If you are surfing, Great Whites means Game Over.
If peeing in the Amazonas, The Candiru Toothpick Fish would be something to dread.
If in Australia, snakes, spiders (black widow) and scorpions would be the worst, since you are so close to them.
If you are socializing in towns, one must avoid The Big Nosy One at all cost.