Mine was Science. We had to take double Science at GCSE, and had a lesson every single day. Five hours a week were wasted sitting in a classroom just messing around because I had absolutely no interest in the subject - and I'm obviously not going to try when a lesson bores me so much. The annoying thing in the end was I failed the coursework, and did pretty good on the exam, and that was without knowing a thing we had been 'taught' over the two years, had I done well on the coursework I could well have got a D, possibly even a C.
After that, I'd say French. In year 7, 8 and 9 I loved it. But in year 10/11 I just lost all interest, and only really picked it because it was the only lesson in the 'block' we could choose from that was better than the rest of them. I had a falling out almost every lesson with the teacher. I did quite well on the coursework, but my exams - due to lack of preparation and revision - brought my grade down to a D.