The most difficult subjects in universities vary from person to person. Different people have different talents and interests, so they may find different subjects the most difficult. Here are some topics that people generally consider the most challenging:

Mathematics: Mathematics is usually considered as one of the most challenging subjects, which requires good logical reasoning ability and mathematical skills.

Physics: Physics requires a deep understanding of the basic principles of nature and a high degree of mathematical and experimental skills.

Chemistry: Chemistry requires a deep understanding of the periodic table of elements, molecules and reactions, as well as a strict grasp of the safety and skills of chemical laboratories.

Biology: Biology needs to deeply understand many aspects of life system, including molecules, cells, tissues, organs, ecosystems and so on.

Computer science: Computer science needs to have a deep understanding of the basic knowledge, algorithms, data structures, programming languages and other aspects of computer science, as well as a full understanding of the working principles of computer software and hardware.

Generally speaking, everyone has different talents and interests, so everyone may find different subjects the most challenging.