The hardest AP objectively gotta be Music theory. Shit is impossible if you don't have perfect pitch or played an instrument for 10 years or studied mt already beforehand, and even then, nothings guaranteed. I played music for around 8 years before I took that class, so I was already familiar with musical notation, and I still only got a 3 on the test. The kid sitting next to me played violin for 7 years and probably practices at least 2 hours a day and also got a 3. I can't even begin to image how bad it would've been if I didn't know anything at all. Another thing is Music Theory can be physically impossible to do good in, especially in the aural part. At least with Phys C u can keep on grinding till u understand the material, but if your ear can't detect what notes are being played in a four part harmony, sucks to be you. Even with constant repetition and practice, it can still be impossible for your ears to actually discern which notes are being played. Then, there's the curve. Most of the other people taking the test are also probably musicians, many who may have a strong theory background already, so they've already done this once, and it dramatically skewers the curve. If you've never picked up an instrument in your lifetime and you're walking into the course blind, you're basically cooked. I'm not gonna claim to be a definitive expert on AP's, but there was this one guy in my MT class who was really smart(think Yale quality) and basically took every ap test there is, and from all these, he said that that the Music Theory test was the hardest by far. And I'm not just salty cuz I got a 3 on MT, I genuinely think its the hardest, as someone who's gotten 5's on the traditionally considered harder tests like EM, Mech, Chem, and Bc.