I think the worst would be Pathfinder Sorcerer, Aberrant Bloodline. You would literally turn more and more into a freak over time. Even as early as 5th level, you grow long weird wiggly arms. And by 20th level (I think?) Your body has become so messed up that you're immune to critical hits! Think of how messed up that would be, slowly turning into an alien creature. Your life would become a body horror story essentially. I would rather eat week old moldy old rolls than endure such a fate.