During my years of teaching, I spent most of my time teaching Algebra. But there was also considerable time spent on Geometry as well. Although, with my Special Education Certification, I have also taught all subjects to students with learning disabilities. While working at the Texas Juvenile Justice Department, I also taught a lot of GED prep classes, which includes Language Arts, Science, and Social Studies. I prefer math, though, because it is objective and sequencial. Math is learned in a specific order. What you have already learned, is used to help you understand the next part or your math journey.
The way I like to explain math, is to tie it to something else about math that you already know. All of math links together. You don't learn to factor before you learn how to multiply. All math has a previous step to it. Understanding that previous step makes new learning easier.