PE. It's not even that I dislike physical activities but being ordered around and to undress and shower with strangers just never felt right to me. It doesn't help that it often leads to bullying and that this particular teaching position tend to attract the worst kind of people. Now that I'm an adult I understand that moving around is important for young people and kids but I also think schools (in most countries) play too big of a role in raising kids outside of academics, certain things can and should be left as the parents responsibility.
I didn't have anything against any other particular subject, I've always loved learning.