The "hardest" major depends on your opinion. Personally I would say math, engineering, physics or chemistry are the hardest majors, because I'm not especially good at those subjects and don't enjoy them. I can write a 20 page essay no problem, but I can't do a problem set to save my life. On the other hand, at my school there were a handful of required writing classes, as with most colleges. I knew an electrical engineering major who could blaze through supremely difficult problem sets, but was just incapable of putting together a passable essay. He flunked (and had to retake) each of the writing classes multiple times.
Pure math at a top school is an incredibly difficult major. Engineering can be passed at a mediocre school if you work hard enough. You can work as hard as you want at pure math at a top 10 school, and still not pass because it's so cognitively demanding.
but some of the greatest economists subscribe to failed policies so it isn't just "kiddy level" v. advanced econ. it's always sold as a science but is really just a humanities branch with stochastics and other tools thrown in. figure how bad "professional" economists are at predicting just about anything using whatever math tools they can muster.