I would say language, but we learn that automatically from childhood.
The second option would be maths, because we use it more than most other stuff in our day to day lives.
We can live without officially studying science, social studies, arts, etc. more easily than than we could live without studying maths. Maths is used for doing any monetary transactions which make a very important part of our lives.
I like to blog about various topics ranging from education in IITs to social issues in India and US. All opinions and views presented here are my own and not of my employer.