It really is debateable and some are most useful in different areas.
For me it has to be history, not only has my essay writing skills improved immensely but a few thousand words seems like nothing. Coupled with that is the fact it has give me great analytical skills, skills involving reading between lines and interpreting, piecing together, presenting a flawless argument, finding flaws in arguments, understanding politics, predicting politics, predicting results, predicting events plus much more.
It's really quite crazy...