I believe the most important math skill to know is solving word problems aka applications. Most good paying jobs will test your word problems skills. Most standardized exams will test your word problem skills.
Employers are not interested in your ability to solve linear equations like x + 4 = 10 or y - 10 = 20. Exams want to know if you know how to reason your way to the answer. I stuggle with word problems more than anything else in math. What is your advice to me? I want to "master" word problems at the high school level at most.