In elementary school it was PE. Absolutely exhausting and I couldn't physically do some of the things they asked. One day after I complained too many times the teacher punished me by having me sit in the corner while the other students ran... and I just did that on purpose for the rest of the year because I wanted to skip the class.

In middle and high school it was science. I actually loved the subject matter but had terrible luck with professors.

I don't have a least favorite subject now! I get to choose my classes as a college student :D