wouldn't we all argue our own major choice was the hardest?
HOWEVER. i really do think my undergrad major was one of the hardest, most time consuming choices if not the hardest - cybernetics. i had to take aerospace engineering courses with the aero majors & biochem with the chem majors and on top of that i was fulfilling my electives with graduate courses in biomath, etc. add a little bit of computer science, neuroscience, upper division math, honors requirements and a research thesis to top it all off. hardly anybody in our program graduates in four years, and we have more required classes than any other science major on campus. i look back now and realize that i had it MUCH worse as an undergraduate than as a grad student now.
then again, i wouldn't change a thing.