The worst I've encountered globally was the Stinging Tree in Australia. Also known as Gympie Gympie. It has a neurotoxin that hurts like a MoFu and you can have some long lasting pain months and even years later that will be reactivated by it from random actions like water splashing on your skin or getting a touched where it stung you. I have encountered thick swaths of this plant while hiking through the Daintree Rainforest in Northern Queensland, Australia. It's a must know plant. Another gem that was in the same area was the Waitawhile vine. It had barbed fishhook like thorns that would grab into your clothes and skin and hold you fast until you ripped them out.