I actually think Rumble's the most dangerous. I know only Dipper faced him, but he's an unstoppable killing machine who doesn't know he's actually doing wrong. He'd "challenge" everyone in the world to become the ultimate warrior if Dipper hadn't fought him, because that's all he knows. Definitely would be scary if it weren't for the bazillions of hilariously awesome references he embodies.

If you're a teenager, the ghost are toughest, because they won't listen to you and WILL kill you. My pick is the wax guys, though, because they don't discriminate, and if they don't like you, won't stop. They're like a zombie swarm. A smart, fast, camouflage zombie swarm. Summerween Trickster might tie them for danger, but he's still only one monster, and only goes after a specific group.