Attempting to categorize subjects of knowledge is not entirely easy, but certain things are clear. All degrees are not intellectually equal, despite what politically correct people would like you to believe. A person studying history and a person studying computer programming can each acquire a vast amount of knowledge of their subject that the other person has no clue about, but in general, most people would consider computer programming more intellectually difficult than studying history. Some of these differences are quite vast, so much that a PHD in an easier subject is much easier to get than a bachelors in another. But it should also be noted, that just because a certain degree may require little intellectual effort to get, it may still lead to quite a difficult career in the real world. Sales and teaching are some of the easiest degrees to get, but may very well be two of the most difficult careers. It's also difficult to include all subjects, particularly since few people (if any) have taken every subject there is and have no idea how hard or easy a certain degree path is. I myself am particularly puzzled regarding the difficulty of architecture. Many subjects I have in the list below I had to totally guess on, but based on the little I know regarding courses of study, here's where I'd rank fields of study in terms of difficulty, with quantum physics at the top:
Quantum Physics
Electrical Engineering
Neuroscience
Medicine
Molecular Biology
Organic Chemistry
Chemical Engineering
Computer Programming
Mechanical Engineering
Civil Engineering
Law
Mathematics
Foreign Language
Accounting
Economics
Philosophy
Nutrition
General Biology
Music
History
Literature
Political Science
Management
International Relations
Industrial Arts
Graphic Design
Journalism
Cultural and Religious Studies
Criminal Justice
Social work
Psychology
Marketing
Education
Fine Arts
Physical Education
Quantum Physics
Electrical Engineering
Neuroscience
Medicine
Molecular Biology
Organic Chemistry
Chemical Engineering
Computer Programming
Mechanical Engineering
Civil Engineering
Law
Mathematics
Foreign Language
Accounting
Economics
Philosophy
Nutrition
General Biology
Music
History
Literature
Political Science
Management
International Relations
Industrial Arts
Graphic Design
Journalism
Cultural and Religious Studies
Criminal Justice
Social work
Psychology
Marketing
Education
Fine Arts
Physical Education