I'm pure math, so I'm not sure how hard it is, but if I could guess, I'd probably rank the (very general) tiers as so:
1. Engineering/Physics/Maths
2. Other Sciences (Bio, Chem, [Pre-med], ...)
3. The Other Other "Sciences" (Geography, Environmental Science, Psychology, ...), Humanities (English, Phil., Langauge majors, Ethnic majors, Poli Sci, Business, ...).
This is all of course judging by the standard courseload for the majors. Obviously, if you do something like 36 units/9 classes of a humanities per semester, it would be harder than 3-4 engineering classes (but I might still argue with that ;D). But it's comparing similar unit-loads.
D H said:
The hardest major is that whose students you never see at parties, you never see hanging out with friends, you never see period. In my school, it was architecture students. They crawled out from their nerdy digs once or twice during the course of a semester, then partied like mad once the semester was over.
Meh. I know about 5 architecture majors and they're all practically hipsters who socialize a lot but never party (because they're hipsters). A lot of them had to spend tons of nights at 'studio', but that was often because of procrastination and/or a need to 'perfect' the project, up to subjectivity of course. And some people are just better at drawing/expressing art than others. Some take just a few hours for projects and others entire weeks.
But I'd agree that it's easy to tell which majors are typically hardest by judging which ones stay locked up studying all day. And in Berkeley, that happens to be the engineers, physicists, and mathematicians. The brightest overachieving students tend to be in those 3 fields.