The nastiest plant in the world?

Last week in Australia I was introduced to the stinger tree or gympie-gympie (Dendrocnyde moroides) , which is common in rainforests after a cyclone or other disturbance.  It is a relative of the common nettle and doesn't look that bad, but is much more vicious. The pain from even a minor sting is intense and can hurt for a year afterwards. I was sorely tempted to give myself a "light" sting just to know what it felt like. Surely a light touch from any plant can't be that nasty? Judging by this video, I am very glad I didn't. Is this the nastiest plant in the world? And why would a plant end up being so overdesigned to subject such overwhelming pain?