What subject in school have you found to be the most helpful in life?

I would say math and English.
Math because I use it so often when I bake, pay bills, figure the per pound or per unit cost of food, figure my taxes,  determine what size container I will need for a project, or customize a pattern.   Algebra was helpful because it helps strengthen logic skills.  Higher math because it helps develop thinking outside of reality-thinking theoretically.
English because I use it every day, in fact I am using it right now as I write this blog.  Without studying English, I wouldn't be able to post this in such a way that the reader could understand.  The grammar wouldn't be correct and certainly, the spelling would be questionable.
Foreign languages are important too because they help you understand a different culture.  You can learn a lot about the people by the words they use most frequently and the number of words they have for a certain thing.  Like in African languages they have many words for dead not like us when we use other words to soften the meaning but they have different words for dead,  one for dead and another for almost dead and another for probably dead etc.
History so we don't repeat our mistakes but learn from them.
Science to explain how and why things do what they do.
Speech so we can explain ourselves or a topic in front of others without panicking.
Music for the joy and the mathematics of it.
Home Economics if our parents didn't teach us how to cook or maintain a house.
Physical Exercise to keep our bodies working as well as our brains.
Study Hall to help us discipline ourselves to use time wisely.
I enjoyed school and most of the subjects except typing.  I never mastered it until later in life when it was part of my duties at my job.  What a difference a dollar makes.