This is one of those questions that will result in a wide variety of thoughts. It all depends on the student. For some people, hard sciences like Chemistry and Physics would be extremely difficult majors, while others struggle with math and would find majors in that field very difficult, etc.
In terms of majors that involve a great deal of time, many would point toward things like engineering, pre-medicine or Biology, other hard sciences, math, nursing and other majors not generally found within the "liberal arts" colleges of major universities.
Again, those, this is purely based on observation and opinion. To really get an idea about the difficulty of a major, check out the program requirements online. Most universities and colleges offer the option of actually taking a look at the number of credit hours required, the courses offered and descriptions of the courses. Compare different majors against each other and the difficult ones, for you, will surely become quite apparent.
I can't imagine many disadvantages to learning English. English is one of the most prolific language in the world, with a lot of the major/most wealthy countries using it as their native language. It is however among the most difficult languages to learn.
Hayden D major cello concerto. Many cellists are unable to perform it.
more difficult, most difficult
more difficult, most difficult
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Difficult: more difficult most difficult
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more difficult, most difficult
more difficult, most difficult
More difficult and most difficult. With a word that long we don't generally add the -er or -est suffixes.
The most difficult question is the one not spoken.