I'd say math just for all my childhood suffering lol, but in retrospect what was bad about it was the dry and unintuitive teaching that never felt applicable to real life, which is a criminal thing to do to any suspect and destroys interest... For example I didn't mind it in science subjects nearly as much even when I struggled with it because at least there were understandable contexts in which the math was applied. At least more than maths.
Since people brought up Religion, I find it funny - despite being atheist even at the time I actually generally enjoyed Religion as a class. It felt easier and more relaxed, and one year we were taught about other major religions and had discussions on Ethics, which were acrually really interesting classes I still remember fondly to this day. (Our teacher kinda went off the curriculum there - despite herself being Orthodox Christian, she spent more time teaching about Islam, Hinduism, Buddhism and Taoism than the deeeply Orthodox ministry of education would expect of her because those religions were also interesting to her and she didn't want to just glance over them. There's mentions of those in the textbooks but brief and in practice they're not really covered by teachers... and the year we were taught Ethics, the formal subject was called Christian Ethics, but she 'secularised' the year by instead encouraging debate on things like e.g. suicide in class, and brought up literature like Brothers Karamazov to discuss ethical questions. Do I sound like I adored her as a teacher? It's because we all did. We convinced her to not retire for a couple years more from the outpouring of written support she got when she announced her retiring. God, I miss her classes.)