I'd have to say the Maricopa Harvester Ant. It has some of the most toxic venom out of all insects in the world, nearly 20 times that of a honeybee. So it's quite formidable even if you find only a few around you, as a sting can cause immense pain for up to four hours.

But if you encounter large numbers of them, you best get away as quickly as you can! Once one of them stings you, a pheromone is released to alert the others and they will subsequently sting you as well.

Seeing as you could die from only a couple of hundred stings, I'd say it is extremely terrifying in large numbers!